Warning; if I'm right this is a spoiler. If I'm wrong though, it is not.
First, some background:
So the Baratheons are pretty well dead right now. So what happens to their little "kingdom"?
Well everyone thinks Tommen is half Baratheon, so it goes to him, pretty much.
The Lannisters are not doing well either, so he would also likely come into possession of their land soon enough. With Loras locked up and Marjorie married to Tommen, this focuses the power quite a bit. Consider as well that Dorne is not doing so hot in terms of legal succession and you could craft an argument that Tommen, as King, is ruler of all of these lands.
Meanwhile Littlefinger controls the Reach, and soon enough, the Riverlands, while Robin, who is related to Sansa, is the figurehead. Sansa's blood also entitles her to the North. With all the shit going on in the iron islands, one could construct an argument that she could rule them as well.
While some of these arguments are a stretch, they are important. Why? Because with a few well placed deaths (all men must die) you can very easily get yourself into a situation with one simple answer:
Tyrion and Sansa end up on the throne.
Martin does not write fairy tails, so I doubt Jon Snow and Dany will end up on the throne, but Tyrion and Sansa are already married, and Tyrion was just about the only man in her life to not try to take advantage of her in one way or another - as such, this is my prediction for how the story ends.