My thoughts on the "bittersweet" ending of the series is that I can totally see the White Walkers continuing to push south through to Kings Landing, and eventually over-running Westeros. I can still see the "heroes" of the story winning a large victory (maybe the Night King or whoever he is, is defeated, but Westeros is lost).
The survivors, whoever they may be, flee across the sea to Mereen, where they are ruled by Dany and her council. That would seem kind of bittersweet to me. Dany gets to rule as a fair and just Queen(yay!), but their homeland is lost forever (boo!). All of Dany's trials and tribulations outside of Westeros have done nothing but teach her how to lead. She's learned the cost of leadership, and how heavy a crown can be. I feel she's a natural ruler, even if young and naive sometimes. I think that's the whole point of her arc. We've seen her grow into a queen, and make both amazing and foolish decisions in the process. I can't think of anyone else in the series that would be a rightful fit in almost all areas. Nobody is perfect, but between the options, Dany is one of the best fit. Maybe Tyrion and a more experienced Jon Snow.
I think losing Westeros completely to the White Walkers would be more cutting than the deaths of our fan favorites. All the betrayal, back-biting, murdering, and war would have got them nothing, but cost them everything (their home). I think that'd be an awesome, powerful, and definitely bittersweet ending. "Revenge and Greed are bad," seems to be the overarching theme of the series. Having that lesson be learned in a way that everyone, not just our lead characters, feel could be great. I don't know. That just a thought that's been in my head for a few months now. I'm very, very interested in seeing how Martin and the show come to their conclusions.