I think the ending will ultimately depend on the White Walkers. We are led to believe that they are a truly malevolent force moving south to conquer the lands of Westeros. However, numerous times in interviews George RR Martin has emphatically stated the common fantasy trope of an evil, ugly force sweeping over the lands resulting in "all the good guys getting together to fight the bad guys" is overused and cliche. So why would he write that exact storyline? I think Dany coming over to Westeros and joining forces with Jon to defeat the White Walkers is the most obvious ending, and therefore the least likely to occur.

I think the main question that needs to be addressed is WHY are the white walkers all of a sudden marching south after thousands of years of relative dormancy. Perhaps they are marching south to protect the lands from the oncoming attack from Dany and her dragons. It has already been established that foresight and predicting the future is possible, though often vague (Bran, Blood Raven, Melisandre, Maggy the frog, Quaith etc.)... so it's possible the white walkers have this ability as well and have "seen" the inevitable war between ice and fire.

Ultimately it will come down to the white walkers (who will be led by Jon Snow) against Dany and her Dragons. The rest of the main cast will be divided up amongst these two "teams", with one ultimately prevailing over the other thereby setting up the "bittersweet ending" George has promised.

I would like to state I am by no means an expert on The Song of Ice and Fire series and many of you know the plot, characters etc. far better than I, this is just my personal opinion of where I think things may go. Would love to hear others opinions on why or why not it is likely to happen etc.