It's going to happen, the formula is pretty simple.
Have a charismatic and selfish warmonger lead all the most glory thirsting warriors to ruin or disregard them when there's no use for them so that they may ruin themselves. Meanwhile back on the islands have a catastrophe brewing that will wipe them out if they don't flee, forcing those left to turn to sensible leadership and having those that refuse fall to the catastrophe.
Euron will use up and burn those he has with him. With the white walkers on the way the Islands will have to turn to the leadership of Rodrik, Asha and Theon or perish. Those that do will live and so will be a new Ironborn, fashioned by Asha and Rodrik as per their conversations. The old way will die, their new way is all that will survive.
Dany will fall, and ultimately she will be the cause, but with a lot of help.
She will kill Aegon and Westeros will hate her for it. She will sentence Jaime to death for murdering Aerys, after he confesses the circumstances. The High Septon will refuse to crown her, and if she doesn't execute him he will otherwise die and she will be blamed. Arianne will likewise die and for it Dany will be blamed. She will use her Unsullied as a slaver would use them, as though their lives are meaningless.
She will be decried, the mad king come again, his daughter returning to finish his work. Her ways of fire and blood will alienate almost everyone, those closest to her will betray her, Tyrion, Barristan and Missandei, for no reason other than they will stop believing in her. The Faith will denounce her and the people take up arms against her. They will force her to flee her throne with almost nothing, poor and dragonless, with only a handful of followers and Jorah.
Reduced to almost nothing, she will examine herself, her actions, her leadership, and conclude she was to blame for her fall, and from there she will rise to her calling, not to rule, but to protect and save. She'll go North, pledge allegiance to her king, go South with him, get her dragon back and help save her realm, probably dying gloriously in the doing.
It will be a civil war, Royce vs Royce, Harry's or LF's death will kickstart it, Yohn will come in force to take SR who Nestor will not surrender as SR is his only bargaining chip to maintain the Gates of the Moon. Yohn will force Nestor back who will retreat back up the Eyrie before Tyrion conveniently arrives on Viserion and sorts the mess out and takes back his wife.
Tyrion is the king Jaime will make when he full turns against Cersei. Jon comes South and defeats Jaime as the first Young Wolf did and the Winter Wolves came South and defeated the Kingmaker. Knocking off Lancel, Deicide, will be his hardest decision.
One of those I wouldn't do it so character is stupid/unrealistic topics. Balon had a plan, it didn't include Theon taking WF, Balon did not fail, he died after barely beginning his war which was abandoned by his successor.
Obviously. Meaning that the total number of people required to carry out a plot by poisoning the pie would be two, the plotter and the servant, assuming the servant is not also the plotter. In comparison the going theory requires a minimum of three people involved.
Conservative. A theory must both remain consistent with and serve theme and character arcs. For example, HR = HS serves no theme and is inconsistent with how the Reeds and Crannogmen are represented. These astrology theories serve noting, they're just superfluous. In comparison, Jojen paste is completely consistent with established theme and serves Bran's arc. The text will also almost always point you in a direction of a mystery that will require an explanation. Examples include who is Lemore? How much truth is in the Pink letter? Or where are the Reeds?
It does not follow that if Tyrion's piece was poisoned that the whole pie must be. Poison could have been added by the servant that carried the pie. It's also an assumption that the cut pie was the same pie from which they all ate. Ceremonial pies and cakes of the like that birds fly out of are often just that, ceremonial, it's common that they would have such a pie/cake and then serve from a pie/cake in the kitchen.