Here are my theories for the final seasons:
First we'll have the Clegane bowl. Cercei will be at war against Daenerys and Dorne. Melisandre will turn against Jon, and join Daenerys. Arya will kill her.
Daenerys is the bad guy in all this. She's gonna become too powerful, and just burn the fuck out of everything. One of her dragons is gonna be killed, and she's gonna go ape-shit.
The white walkers are actually a good thing, bringing balance to things, and are there to extinguish Daenerys' fire. And it's acutally Bran who sees this, and will go back in time to bring in the white walkers, in the first place. He mind-travels back in time to get the children to create them, so they will be ready to stop Daenerys. They will try to defeat daenerys by controlling one of her remaining dragons. Also, the wall was built to stop Daenerys' army of fire, not to stop the white walkers.
Jon Snow will die to stop her. And when all the dust has settled, and pretty much all the main characters are dead, Littlefinger will sit on the Iron Throne.