I believe that we as fans can make our own ending if GRRM is not up to it.
I'm at the final 6 hours of the audiobook, and at Cersei's 'Walk of Shame'. So this is where the book ends (compared to the series).
For me I'm thinking of my ending as being positive/happy/wise in nature as as philosophically and morally ideal as possible. I would like to see as many 'redemption arcs' as possible for the villians and anti-heroes. Basically a Witcher ending.
I was thinking about it the other day. Not finalized or anything:
eg.
-Cersei gets her revenge on the High sparrow and faith like in the show, maybe not in the same way but the threat to the Lannisters is removed. She gets mortally wounded and has a painful, slow lingering death. Jaime spends weeks with her and there is some sort of final 'redemption' and final understanding of her wrongs and issues from the dying bed. With her death, Jaime is free to take command of the Lannisters and is also free of her. He marries Brienne of Tarth.
-Tyrion partners up the dwarf girl, Penny and gets over his issues while being advisor to Dany.
-The White Walkers start to stream-roll Westeros. By Circumstance, The Boltons and their allies are the first to confront them and they get annhilated.
-Stannis and Davos become Jon Snow's mentors and the Jon Snow character grows a lot in maturity and wisdom. They fight the White Walkers but are defeated. Stannis dies a hero's death with Melisandre dies a heroine's death. Both realize that the prophecy of Azor Ahai was wrong. Jon Snow and Davos and the remnants retreat South.
-The Human factions pause their Civil war as it is clear that the White Walkers are worse than anything else. Jon Snow unites the humans against the White Walkers. The White Walkers are defeated but at heavy cost to the humans, and all of Dany's dragons are dead. So this is like 'nuclear disarmament' but in medieval terms.
-Jon Snow and Dany become King and Queen. Jon Snow is not a hidden Targaryen or anything, just Eddard Stark's Basterd. No incest or anything.
-real straightforward ending that would have been way more satisfying than the one fans got in the show, and would have made everyone happy.