Couldn't find the other thread on this, figured I would post this here.

These are just guesses on my part.

Looking big picture, Jon and Sansa have to fail to retake the North, right?
I mean, eventually Dany is going to cross the sea with her dragons and massive foreign army, to claim the throne. That will meet with massive resistance, unless... she is the last hope for Westeros against the Night's King.

To be so, you have to take all the other parties out of the equation.

My best guesstimate: Ramsay wins the North, Wun Wun (a potentially key weapon against the Wight army) dies in the battle. Jon Snow's army is scattered; Jon and Sansa survive somehow. Maybe the Boltons defeat the forces of the Vale too (thus eliminating yet another army that could face the Dead).

Jaime and the Lannisters take out Blackfish.

Dorne engages in some type of attack on King's Landing (possibly assassination attempts).

With the Lannister army in the North, Tommen calls on Margaery to send the Tyrells to retaliate. This effectively ties up both the Tyrells and the Martells.

The Dead come, Ramsay is northernmost, falls first. He may or may not have Bran in his control by then, but Bran escapes south with Jon and Sansa to find some refuge to resist.

Jaime takes the Stark refugees, including Bran. He's smart enough to listen to them (as compared to Ramsay, who didn't see the bigger picture), and he has the biggest army in Westeros at his command. He may or not be swayed by Brienne, and besides, "a Lannister pays his debts". He figures he owes Bran, since he tossed him out the window. Also, Jaime has already shown he will do the necessary thing, even if it means bad for him (see: Kingslayer). So he sends the Starks south (or leaves them where the camp is), and marches North to face the Night's King, probably expecting the outcome.
He, too, falls, but in a far more noble manner than Ramsay.

This pushes Cersei over the edge, and her bitterness over losing Jaime coupled with her perceived betrayal by Tommen (who sent him North and sided with the Sparrow against her), causes her to lash out. Since the Mountain acts on her emotions, the Mountain attacks the Crown/Faith alliance, and in the process kills Tommen and Margaery. Distraught, Cersei commits suicide, and King's Landing is in complete disarray. Sandor kills the Mountain.

The Dead are sweeping south against little resistance, and what little remains of the ruling class (mostly Starks, ironically) have arrived at King's Landing, for a last stand, when Dany arrives. Expecting a fight, she instead finds a broken populace more than willing to give her whatever title she wants, so long as she can save them.

Dany accepts the throne, and takes it as her duty as rightful Queen of the Seven Kingdoms to protect it and defeat the Dead, despite a lot of counsel to withdraw back to the relative safety of Merreen across the sea. She believes she was put through all her troubles to strengthen her for this task.

We see a final battle of the Night's King and his forces, vs Dany and the various religions that flock to her (The Red Priestesses for the Fire God, Arya (and Jaqen) for the Many Faced God, whichever Greyjoy brought the fleet for the Drowned God, Bran for the Old Gods, Sandor and the Sparrow for the Faith).
Massive attrition, and it comes down to Dany and Drogon vs the Night's King. He kills Drogon, and it seems like he will kill Dany and prevail in the end; but a grayscaled Jorah, wielding a Valarian sword (maybe Tarly's, obtained from Sam), takes the blow, and the grayscale takes rapid effect on the King. Jon, the annointed one, finishes off the Night's King, and Jorah dies happy in the friendzone.

Dany takes the throne of everywhere (Westeros, Essos etc), with Jon as her Beta King, while Sansa and Tyrion Lannister are given Winterfell and the North.