Bran: By the looks of things Bloodraven is in at least two episodes, so meeting him wont be the season ending, unless they choose to have him in Brans dreams or something. So either have that happen and him actually meeting Bloodraven in the flesh as the series ending or have him meet bloodraven mid season and having the series ending something like Bloodraven revealing who he is, or Bran doing something like going on to the weirnet for the first time.
Arya: After spending the first part of the season trying to get to the Eyrie, and also perhaps introducing Arya as a warg too, have them return to the riverlands and get Needle back in episode 8 or 9 , with the hound getting wounded too. She refuses to give him mercy either in Episode 9 or 10, and Episode 10, the 'valar morghulis' moment in episode 10, so she's off to Braavos for Season 5.
Sansa: Probably introduce the Lords Declarant storyline (or at least the background to it) a bit earlier when Lysa's still alive, should fill up the extra time in her storyline after arriving in the Vale and the LF-Lysa marriage around mid season. The ending though still has to be the 'only cat' moment.
Jon: Looks like Battle of the Wall is in episode 9, hopefully it will play out over several episodes rather than one, with that being the major climax. Still think the best way to do it would be to have the LC election before the end of the season, so its a new chapter in Jons storyline come season 5, even if that means either stannis making his offer in either the same episode as the battle(possible if it ends early in the episode), or, more likely the same episode as the election.
Sam: As in books he manipulates the election so that Jon wins (possibly with the help of Davos, if the show does what i think it will do and have him hang around Castle Black for a while rather than go straight off to white harbour
Iron Islands: Not really sure how the Yara vs Ramsay storyline will end, probably with Yara losing, anyway looks as though that might last the entire season given how they have yet to cast any Greyjoys for the kingsmoot. Yara's storyline might end with her receiving word that her father has died, setting up the kingsmoot for the season after.
Theon: His storyline will probably be heavily involved with Yara's, but when its not i'd like to see some other stuff, perhaps along the lines of a hunt for bran and rickon, or him winning the northern lords to his cause. Anyway, the ending should be him telling theon he is taking him south to moat cailin to meet up with his father.
Jaime: Looks like he'll send off Brienne a bit earlier than in the books, probably at some point after the PW (maybe after he see's fake arya, if he does see her) I think it would be best if Jaime somehow wasnt in command of the KG at the PW, perhaps he leaves the feast to go to see Brienne, because he is sick of his father and sister. For the rest of the season his story should focus on his duties in the KG, his relationships with his family, including tyrion and tommen as well as cersei and tywin, and ending with im breaking Tyrion out and telling him the truth about Tysha.
Brienne: Having been sent out by Jaime earlier than in the books, i reckon they would probably end the season with her killing Rorge (presumably minus Hound helmet, given the Hound wouldnt have vacated it by that point in the show. Although, seeing as how i think its likely Podrick will turn up with her at some point this season (otherwise she has no one to talk to) , i wonder how they will get him out of KL before the end of Tyrions Trial (or alternatively her meeting up with him could be the season ending if Jaime lets Brienne go towards the end of the season but with two or three episodes to spare. However, i think first option is more likely given that Rorge is returning this season. Alternatively, I wouldnt mind to much if they changed this storyline up a bit from the books, as it was probably the least interesting out of all of them.
Tyrion: As in the books, kills Tywin, Shae, delivers some line about Tywin shitting gold, and hops on a ship across the narrow sea.
Daenerys: Takes Meereen about mid season, decides to stay there for a while, the rest of the season is mainly about the Jorah and Daario subplots coming to the forefront (although see a couple of plots for next season begin to surface), in episode 10 she sleeps with Daario for the first time and/or banishes Jorah.
Roose& the Freys: Get occasional scenes with Roose and Walder, or Roose with Fat Walda, and stuff like that, throughout the season. They mainly concern stuff like Roose planning how to retake the north while he waits for his (f)arya to arrive, and also some stuff with the Freys, throughout the season there are reports of outlaws and 'lady stoneheart' doing bad stuff to the Freys and their soldiers. Ends with one of them being captured, Walder sends one of his sons to pay the ransom, setting up the final scene of the season as the epilogue of aSoS. By the end of the season, Roose has received (f) Arya, and is marching north.
Note that I agree with D&D that there should be an extended season 5 that covers most of aDwD and aFfC in one season, should last roughly 12 episodes
Bran: Ending Unknown. Probably spend most of the season on elaborated aDwD stuff, mainly his training in the weirnet from Bloodraven (guest appearance from Sean Bean anyone?) But they would probably want to end the season with some further plot development from early tWoW, if there is anything.
Arya: This season would be all aFfC/aDwD stuff. Start with her going into the house of black and white, meets Sam and kills Dareon about 3/4s or so through the season, and blinded after that, regains her sight couple of episodes later, and end with her killing the man with the coin.
Sansa: Dont know how it should end, first part though should begin though with LF taking measures to cover up how Lysa died, and then continue on to the Lords Declarant mid season, then we find out about LFs plan concerning Harold Hardyng (if that is really what will happen, which i'm not so sure it will be) and then maybe Sansa meets him at the end of the season, or that might be a bit early for season end, so maybe 2 or 3 episodes before the end of the season.
Jon: Most of the season about Jon being LC, along the lines of his aDwD stuff, trying to reconcile the wildlings and the NW, trying to deal with stannis and stay out of the Northern War, and trying to prepare the NW for winter whilst facing opposition from a lot of them. Anyway, the wildlings come through the gate episode 9, with Jon getting Julius Caesared in episode 11 (if he comes back that is) so the unsullied will get some hope, if not outright proof, that he will be okay (so all of them dont quit the show), and so to fuck with people's minds when they show the Battle of Ice the next week in the final episode (if stannis wins that is)
Stannis: Goes south around episode 3, after following Jons advice. Couple of episodes later he wins the support of the mountain clans and captures deepwood motte soon after. Sets out for Winterfell following episode, and from then till the last couple of episodes his situation deteriorates as his army gets caught in the blizzard. Stops in early Episode 11 at the place he currently is in the books, but Jon receives the pink letter at the end of the episode to say stannis had been defeated ( a lot of unsullied might buy it that the show would show it this way rather than pay for the actual battle) and then everything from theons first tWoW chapter onwards would be in the final Episode 12 the Battle of Ice.
Davos: Sent off to White Harbour in Episode 3 when stannis goes south, Sallahdor Saan abandons Stannis after he drops Davos off at White Harbour (getting rid of all the stuff about the sisters) Goes before Wyman Manderly mid season, that episode ends with us seeing a head that looks a lot like Davos' put up on a spike, but he turns up a couple of episodes later when Wyman Manderly tells him his plans, leaving the final third or quarter of the season for him to spend go finding Rickon (unsure if GRRM is actually going to include this in the books, but the show should)
Sam: Arrives at Oldtown, or if not an attack by the Ironborn on Oldtown (if there is one)
Iron Islands: Kingsmoot done by mid to early late season, Victarion sent off earlier by Euron too go find Dany with the Iron Fleet, Fishes out Moqorro in the final episode of the season.
Theon: Taking of Moat Cailin episode two or three, arrival at Winterfell in episode 6, escape with fake arya episode 10, see's Yara again in episode 11. Episode 12 Battle of Ice.
Jaime: Sent off to the Riverlands episode 6 roughly, wraps up everything there by the end of the season (trains with his left hand in the meantime, possibly with Bronn), when Brienne comes running and tells him he needs to come and help her rescue Sansa from the Hound.
Cersei: Basically does all the stuff she loves to do when she rules, piss off the tyrells, be paranoid about Tyrion, and just generally lead westeros into the toilet, episode 9 Margaery is imprisoned, and at the end of the same episode the High Septon imprisons her too. Walk of Shame Episode 11, and then she makes a brief appearance.
Dorne: Maybe introduce Arianne's affair with Arys Oakheart from the beginning of the season rather than just have it already happen when the season starts, it gives time to get to know the characters. By Ep3 or 4 she has convinced him to go with her plan, which is defeated ep7 or so. Arys dies, Arianne spends the next few episodes in the tower, in Ep10 she has her conversation with Doran when he reveals his intentions, and then in Ep12 having him receive the mountains head, before he sends off the Sand Snakes and Arianne.
Tyrion: Smuggled into Illyrio's mansion episode 1. On board shy maid by episode 2. Episode 5 is the stone men and his discovery of who Aegon is. Episode 6 he is kidnapped by Jorah, ep8 there is the storm on the ship and it gets boarded by slavers the next episode, sold off in the next episode joins the second sons by Episode 12
Aegon: After getting seperated from Tyrion in episode 5, they meet with the Golden Company in Episode 7, in the Seven Kingdoms by Episode 10, if they include the bit about Griffins Roost then they should do it at the beginning of the following episode, if they don't then they might want to have them take Storms End by the end of the season rather than leaving it till the next
Daenerys: Picks up where the aDwD elements of the plot left off last season, proceeds along much the same lines, agrees to marry Hizdahr in episode 7, wedding in episode 9 (may need to put the whole storyline back a couple of episodes if they feel the need to have Tyrion perform there) With Dany reappearing Episode 12
Barristan: Ends with capture of Hizdahr and death of Quentyn.
Kevan: Increased focus towards him as a character as opposed to the books, especially in late season, when we get to see him sort out the realm after Cersei's capture in Episode 9 rather than just hearing about it, that will also help increase unsullied reaction to when he dies in episode 12.
In season 6 they could have siege of Meereen in the opening episodes in order to avoid losing one out of that one and the Battle of Ice due to budget issues. Cersei's trial by combat and other stuff pushed back to then too.