Season 6 wasn't as good as people think it was. It was just better than s5, s7 and s8. But s6 had some pretty big problems that only were shown in full-effect in season 7. If I had it my way, I would go back and rewrite s5 and s6 as well. But that's not being on-topic.
How would I have done season 7 and 8?
Well, the first thing I would've done is let season 7 and season 8 have the full 10 episodes that had become standard. I actually might've made the last season longer by giving it three more episodes.
The nature of the Long Night demands that an entire season (if not two) needs to be like The Walking Dead...except with lots of snow and ice, dragons, magic and an endless cover of darkness. An entire season (except for maybe the end of the last episode) would need to be filmed at night and/or in candlelit dark rooms. It doesn't mean that it has to be so dark and murky in that no one can see what is happening but it must be communicated that the world has been covered in darkness and that the light that would come from the moon, the sun and the stars would be either completely absent or unusually weak. Proper lighting, camera work and post-production can accomplish that.
As far as actual content: I'll start with...SEASON 7
Let's start with the south. D&D didn't only fuck up the plot. They made a huge mistake after huge mistake in character management. The story in northern Westeros in season 7 (Littlefinger putting Sansa and Arya against each other) wasn't a bad idea but the story in southern Westeros in season 7 was trash. There was literally nothing for anyone to do there. No tension. Like the Lannisters vs. Daenerys isn't even a contest.
I would actually leave a lot of what happened with the Lannisters the same in season 7. I make it clear that Tyrion wants to be the head of House Lannister which explains why he has such a incompetently gentle approach. He doesn't want to damage Casterly Rock or destroy Lannister armies and supply lines; he doesn't even want to be a kinslayer anymore. He only wants to remove Cersei from power and have her and Jaime put under the most luxurious house arrest or otherwise out of the way (he might want to send Jaime to the Wall and Cersei to a motherhouse).
Neither Jaime nor Cersei would find it comforting though.
Cersei can blow up the Sept of Baelor with wildfire (that whole spiel was a very in-character moment) but no one would want her. She would still have power over the West but they are on the other side of the continent. She will be rapidly losing power and support in King's Landing though.
I would have delayed the alliance between the Lannisters and the Greyjoys so that Cersei could face off with both Aegon and Daenerys. Daenerys, despite all of her advantages, would be fighting off Euron, Aegon and Cersei. And I would have Aegon be a figure propped up by the Faith which would still be a much more aggressive, militant force after Cersei killed the High Sparrow.
I might separate the Lannister twins at some point and they would lose King's Landing and be forced to return to Casterly Rock (i.e. there was no point in featuring Casterly Rock in season 7 the way it was) either by episode 4 or 5 in season 7. From there, Cersei would organize an alliance with Euron. Euron captures her on her way back to the Rock but Cersei is able to make the best of it. I would make it so that Jaime is the one who hires the Golden Company but that Cersei and Euron not only appropriate the Golden Company for their purposes but that they hire a crew of Faceless Men.
Gendry would be my boots-on-the-ground POV what with all the upheaval and chaos. I'd show what Gendry had been up to since season 3. Maybe he had a wife and a child that died when Cersei nuked the Great Sept.
As for as who would have control of King's Landing....
fAegon and the Martells
The Iron Bank hedges its bets on Aegon. However, they are seeking a financial takeover of Westeros which would be the end of an era. Most characters are pro-feudalism or pro-absolutism so they are fiercely opposed to this idea. But Aegon and the Martells would not have a problem with it.
To this end, I would've had Arianne and/or Quentyn appear. I would probably have to do some retconning and ass-pulling to make it work because Trystane is passed off in previous seasons like he is the only son of Prince Doran. I don't remember if that was actually established but it's Dorne so...hey. I can do whatever I want. If someone dislocates their shoulder, relocating the shoulder is painful and unsavory but necessary. So what's one more ass-pull?
I likely would have Arianne and Quentyn find shelter in Starfall with House Dayne. Wylla, Allyria Dayne, Ned Dayne and Gerold Dayne would all be in the series and Ashara and Arthur would likely appear in more of Bran's visions of what Ned did at the Tower of Joy and then immediately after.
I would have Arianne stay in Dorne to try and retake it from her bastard cousins while Quentyn is up with Aegon with the Faith. I'd change Quentyn's character and make him a septon or a knight sworn to Aegon to further flesh out the "Revenge of the Faith" plot...which I would have it so that far southern Westeros is all rallying in defense of the Faith. To that end, Quentyn would not be a sanctimonious bible-thumper like the High Sparrow or Melisandre were...he'd be a much more down-to-earth, kinder religious person. The goals of the Faith would be to put a constitutional monarch on the Iron Throne who would have to work within a system built and managed by a combination of the clergy, the banking system established by the Braavosi and the Westerosi nobility.
Think a much more religious version of the Magna Carta or the US Constitution. Except with three branches of government, you have four with the Braavosi bankers and keyholders being the fourth.
Quentyn would still die by dragonfire but in the show I'd make Dany do it on purpose which would make Arianne even more bitter. Think of the whole "she burned the Tarlys" thing but instead of it being Asshole Randyll and his dumb son it'd be charismatically pious and kind Quentyn. You have a lot more
In the books, GRRM uses Tyrion (and a couple other characters) to drive home the point that some allies are more dangerous than enemies. Ellaria and the Sand Snakes would epitomize that as their alliance with Dany only puts a bigger target on Dany's back as both Team Aegon/Martell and Team Lannister have a legitimate beef.
Aegon is sitting on the Iron Throne however by the end of the season not Cersei.
While Quentyn dies in the wars of season 7, Arianne will survive and go on to play an important role in season 8. Why? Because in the finale, over her course to rally Dorne, she would find out that Aegon is a fake and the real son of Rhaegar was born in Dorne to a northerner and that that son was named Jon Snow. Basically, she and Samwell would be the ones who find the evidence that Rhaegar and Lyanna were, in fact, married.
Since Arianne has no connection with Jon Snow (unlike Samwell and Sansa), this is big news...news that is able to set Westeros on fire. It's a big revelation and Arianne would be part of that. Jon Snow would be like a messiah to them.
Daenerys Targaryen and friends
Since we're having this whole religiously-centered revolution going on, I would have the red priests rally around Dany. This also makes Dany look worse than what she is as many of the characters would be reminded of Stannis and Melisandre. I would have Kinvara travel with Dany to Westeros and fuse Moqorro and Benerro into one character that won't appear until season 8.
I realize the actress who played Melisandre as very pregnant at the time and needed some time off but, for the time that I had her, I would have Melisandre make a big impression on Dany, trade barbs with Varys, Kinvara and Tyrion, be awkward around Jon Snow and Davos Seaworth before being summoned back to Volantis by Benerro/Moqorro.
Before she leaves though, Melisandre would actually be working as a matchmaker between Dany and Jon. Although it was one of the highlights of season 7, Dany and Jon's relationship was played out a bit clumsily and sparsely. Melisandre whispering in their ears so as to glue them together would make for a better story.
I would also make it a big point for Dany to stay in contact with Daario...more on that in season 8.
Other than that, I would make the military tactics of Dany clearer. In the TV show, it seemed as if her forces were teleporting on and off of Dragonstone. It makes sense for Dany to take Dragonstone and house her court and dragons there. It however does not make sense as a wartime seat or a headquarters for a massive land army.
If they wanted to have Dany's forces grounded on Dragonstone, then that's fine. I would still have a Euron vs. Dany moment super early in season 7 in where Euron destroys half of Dany's navy. I might have multiple Euron vs. Dany moments because I would be building up to an intense showdown between the two of them.
In any case, Dany needs a foothold on the continent. And I would have it be either Crackclaw Point or Gulltown (which would give Sansa and the Vale lords a much better reason to panic and try to prevent Jon Snow from treating with her)
So, I would have a scene where Dany takes a few good men on dragonback (why does she not have a Queensguard anyways??!?!) to a castle in Crackclaw Point or in the southern Vale which they take by storm. From there, she can ferry the Dothraki to the mainland where another Field of Fire can happen. I might have her take Sweetrobin into custody.
Varys would be revealed in season 8 to be the puppetmaster of the Aegon machine. He'd probably be trying to get them to marry with Aegon as the ruler and Dany as a consort and arms supplier but his plans would be thwarted by Jon Snow.
I also need more named Dothraki characters. Bring back her bloodriders or incorporate one of the Dosh Khaleen in her inner circle.
I don't have a problem with 99% of the Tyrell family attending Loras' trial and being blown to smithereens. I don't even have a problem with Lady Olenna being the last Tyrell.
I do have a problem with how the Tyrells went out.
As far as land armies go, they are the superpower of the continent and have arguably the best naval force on the continent as well. Even if Randyll Tarly betrays Olenna for Aegon (LOL never Cersei) and steals a significant portion of her forces, the Tyrells should still have the ability to put up a damn good fight so as to defend Highgarden and make it a power base for Dany's forces.
I wouldn't make the Queen of Thorns the last Tyrell standing. I would have her joined by Willas (the baby brother of Margaery and Loras) who is also crippled. Through Willas (and, to a lesser degree, Olenna), I would be exploring the concept of ableism: is a cripple is capable and worthy of being a ruler? Is the mind and soul negatively impacted by physical trauma and handicaps? This would do some interesting things for Tyrion's character and it would also the shellshock of "Bran the Broken, King of Westeros" at the end of season 8.
I'd probably have the Tarly-Lannister combo finish off the Tyrells only for them to be destroyed by Daenerys Targaryen. Willas will die but Olenna is taken captive only to be freed by Dany. While Jaime Lannister and Randyll Tarly are at Highgarden, Cersei loses King's Landing and leaves for Casterly Rock by sea. Eventually the twins meet back up in Casterly Rock (where Cersei has married Euron) where they stay until the Dragonpit meeting in the finale. I'd have them interact with their aunt Gemma Lannister and their cousin Daven at Casterly Rock. Aunt is pro-twincest but cousin is anti-twincest.
Olenna would be gunning to make Willas the consort of Dany. So yes, Dany in season 7 should be having to beat off men with a stick: Willas, Aegon, Quentyn, Tyrion and Euron would all be trying to marry her.
Samwell Tarly and the Greyjoys
For one, since this is a 10 episode season and not a 7 episode season, Sam would be at the Citadel for a longer period of time. Most of his stuff would be infodumps about the seasons. But since these wars in season 7 and 8 are basically religious in nature (or have a strong religious backing), the maesters present another theater to these religious wars. Anti-religion.
The Citadel, as I would have it, would be pro-science, anti-religion and anti-magic to the core. As such, Dany and Euron both would be aiming to silence the Citadel. (Again, Dany - although she is the hero - looks terrible for having beef with Westerosi system of academia but this time they would be trying to kill her and her dragons by sending spies and assassins against her on Dragonstone)
Sam has the additional obstacle of trying to find information about the Others and the previous Long Night without anyone else knowing. Archmaester Marwyn and Sarella would be featured here ("not all Sand Snakes are bad"). I would have Jaqen H'ghar show up to help Sam. As the Faceless Men are opposed to the White Walkers because, to them, the dead should stay dead.
I would have Euron attack Oldtown at some point to also get information. And yes, my version of Euron would be a Jack Sparrow knockoff on the outside but, behind closed doors, a deviously intelligent politician and sorcerer. I would have him torture Yara like Ramsay tortured Theon (and make it so that Theon hears about it)
While I would make it clear that Euron wants information on how to steal dragons and he wants to take the Citadel for his seat, his intentions as to what he wants it for would be kept secret. I would also not reveal that Euron wants information on more than just dragons until later.
Euron's activities: I would have Euron meet with Dany on Dragonstone, then I would have him destroy a fourth of Dany's fleet. Then I would have the Ironborn under his command continually harass Dany over the course of the season...much like how the Yunkish and Qartheen harassed Dany in Qarth. The Ironborn would not only obliterate the Tyrell fleet as well but they would make any form of sea travel in Westeros south of the Neck incredibly difficult. In the meantime, Euron is personally attacking Oldtown. He captures Cersei on her way back to Casterly Rock but returns her to Casterly Rock where they make an alliance...by way of marriage. He returns to Oldtown with Qyburn and a Lannister contingent and takes the city.
Sam escapes with Gilly and her baby. Euron sacks the Citadel, finding the Horn of Joramun, learns that it will void the magic of the Wall when blown and blows it. The blaring of the horn is heard all through Westeros.
Okay, since we have much more episodes, I would delay Arya's return to Winterfell.
Not including Lady Stoneheart was a terrible mistake that doomed the show but, as of season 7, it's too late to bring her in. Arya would take Lady Stoneheart's role in that she is hunting and butchering Freys and the Lannister soldiers there, eventually meeting the Hound, Thoros and Beric here. She becomes the de facto leader of the Brotherhood without Banners.
I would have Arya burn the Twins, retrieve the bodies of Robb and Catelyn (which had been disgustingly put on display in the Twins), frees uncle on screen and then restore him to Riverrun. I would also have her meet Nymeria and her wolf pack. The two would part ways initially.
In any case, Arya gets quite the following in the Riverlands and quite the reputation across all of Westeros but her identity remains mysterious to those in Team Lannister, Team Targaryen, Team Martell and even her family in Team Stark.
After she is done killing all the Freys (innocent or guilty) and before she can start killing all Lannisters, Arya will get word of Jon Snow becoming King in the North...from none other than Bran, speaking to her from a weirwood tree. She will drop everything she is doing, leave the Riverlands and their restored (but small) army to Edmure. She crosses paths with Nymeria again but this time Nymeria and her pack join her.
They go north and cross paths with Meera and Howland Reed who proceeds to give her a clue about Jon and asks her to pass a message on to Bran.
But yeah. Arya shows up in Winterfell at the head of a small army (the Brotherhood) with a pack of wolves and the remains of Cat and Robb. At which point, she and Sansa can start beefing.
Honestly, the Sansa and Littlefinger stuff....that can stay. I wouldn't change much with that other than Sansa's dilemma in season 7 is deciding if she is going to listen to the devil on her shoulder (Littlefinger, the Valemen) or the angel on her shoulder (her family, Brienne, the Northmen)
The Valemen will want to go south to fight Dany and free Sweetrobin but the Northmen will want to stay put in the North. Both groups are annoyed with Jon's departure and want her to become their queen. Littlefinger disbelieves and downplays the stuff about the Others and plants seeds in Sansa's head about her own bid for the Iron Throne.
And Bran? Ah, beautiful, bold Brandon Stark. First thing that needs to be revising is his character: he went from relatively lively yet serious in both season 6 finale and season 7 premiere but then he got really weird and, frankly, creepy in the second or third episode. I would axe the whole robotic thing he has. I would have him be quiet and serious and determined but I would give him a very morbid, sarcastic sense of humor that endears him to the northmen (and eventually everyone else). Bran would still be dropping "I know almost everything" bombs on people like Sansa and Littlefinger but he'd do it in a more tasteful, socially appropriate way.
I would have Bran accept his title as Lord of Winterfell but he would leave a lot of the administrative, logistical and militaristic duties that a great lord would fulfill to Sansa. He, instead, spends most of his time in the godswood spying on and fighting the Others by proxy by using wild animals and trees (yes, I would have Bran make trees come alive as well!!!) beyond the Wall. Kinda like a virtual reality video game. It would make for some really cool sequences.
But I would also have Bran host guests and hold daily religious ceremonies. Not only does this lay more groundwork for him being accepted as a king at the end _- despite his physical handicap -- but it also explains:
why Bran didn't simply nip the Littlefinger/Sansa/Arya issue in the bud from jump (aka he's been busy)
it strengthens this whole holy war arc we have
The Faceless Men is a religious cult dedicated to the god of death and Arya's hit-list is almost a prayer. I would link the Many-Faced God Syrio told her about and the Faceless Men dedicated themselves to to the old gods of the North. So, I would gradually have the North being socialized to revere the old gods at a much more deeper level. I would have Bran explain this to Arya and Sansa. Arya gets it almost immediately but Sansa slowly comes around. This is what unites the sisters. While you have religion tearing the country apart south of the Neck, religion is not only empowering the people north of the Neck but it's healing the rift between the Stark siblings.
At this point, I would have Bran work to unlock his sisters' power of skinchanging. Arya would get the hang of it before the end of the season but Sansa doesn't catch on until towards the end of season 8 or even season 9. More on that later.
The southerners will have likely heard of Bran and Arya. Sansa is already widely respected and feared (it's an awkward surprise for her) because she is widely considered to have both avenged the Red Wedding by killing Joffrey at his wedding and to have been the mastermind behind the downfall of House Bolton. (Sansa embracing the lies that her political reputation is built upon will be part of the reason why Arya hates her)
Nevertheless, over the course of the season, all of Westeros starts developing a deep respect and fear towards the Starks, particularly Bran.
To wrap it all up, the last episode of the season would mostly stay the same. The only characters not present for the Dragonpit meeting are Arianne, Sam, Littlefinger, Yara and the Starks. Ned Dayne would be there on behalf of Sunspear and Brienne and the Hound would be there on behalf of Winterfell. Sam Tarly would have just kept on going north, to put as much distance between him and Euron as possible. Euron and Qyburn leave early for Oldtown to execute his and Cersei's plans of allowing the Others to invade and destroy Westeros. The wight hunt is still present but, as we are talking Aegon, it has an actual purpose and function. Viserion is not lost beyond the Wall because he doesn't go. Dany rides Drogon there but Rhaegal - whom Jon has bonded with - flies to Jon's rescue on his own volition (another hint of his Targaryen ancestry which Dany marvels at, remembering how Drogon came to his rescue). Both the Citadel, the Iron Bank and the Faith try to kill Viserion in the meantime but they are thwarted by Viserion himself, Bran and Tyrion...who only wants Viserion for himself. But Viserion refuses him...for the time being. The Night King instead raises another dragon from the dead (this is what Bran was trying to prevent: the raising of the "ice" dragon) which he uses to physically destroy the Wall. Dany and Jon are officially married, joining the North, the Vale and the Riverlands to her cause...which is controversial to say the least.
Delaying Bran's discovery of Jon being a trueborn Targaryen to season 7 finale was stupid. I would just have him know it all along and then reveal how he knew it and how much he knows in a flashback or vision.
I also would have kept Littlefinger alive longer. Why? I would have just delayed it: end the season with him being exposed and imprisoned and save his execution for season 8. As Jon is king, he technically has the last say and none of the Starks would wish to usurp that authority. This way, Littlefinger can have a brief moment to reunite with Sweetrobin, Varys and Tyrion and corroborate the story of Jon's true parentage by bringing up the truth of Harrenhal before he is executed.
I'll be back for season 8. I hope this was intelligible.