Spoilers, I guess.
I think that any guess is as good as the other at this point. I mean, yes, allegedly the creators had iput from GRRM, to have some idea where the story goes... in broad strokes. The divergence of the series from the books has already gotten so high I don't think anyone's really able to make any educated guess as to what will come next based on the books themselves. For all the energy and time GRRM spent on the Meereenese knot (the importance of which still escapes me, BTW) the show doesn't really follow. When I remember the last two books, there are many scenes/storylines that just weren't in the series. The series also concentrate on different things/scenes.
I also think that GRRM might have underestimated his own tendency to spread - after 5 doorstoppers, not that much really happened, winter is still coming, the Others are nowhere to be seen etc. And his plan is to finish it in mere two books. 5 books of world- and ominous-dread-building is impossible to finish that shortly, unless you want to leave a lot of people disappointed. So my guess is, there will probably be more books. But maybe I'm wrong.