To be honest (spoliers) I can only see the story going one way. Which is that the seven kingdoms bands together to defeat the White Walkers and the Starks renuite.
Of course the plot is a lot more intricate than that, because we're all interested in our favourite characters, but the ending of the series won't be the evil White Walkers destroy humanity.
There seems to be only one way it can go, the only thing is how we get there. I don't think George will die before he finishes because hes not that old and he only has 2 books to go (I think).
If he does I'm sure whoever finishes it will be able to figure out how to get to that ending. Dragons, hint, hint. But I have heard that in the event he does die he dosen't want anyone else to finsih his books. Only 2 to go though, so I doubt we'll be left without an ending.
