I keep thinking about the "bittersweet ending" thing. I'm just spitballing here, but what I'm guessing happens is that it's a "happy ending" in that Cersei is ousted, but nobody is happy because they've lost the people they love most.
I am somewhat predicting Jon dies (either he or Dany is going to, there's not really a way to sustain both) - so if Dany gets the throne, it's bittersweet because she's ruling over a land where nobody loves her and she lost the only person who did. If Dany dies and Jon rules, it's him ruling a land he loves but doesn't want to rule without the person he loves. I'm thinking either Jaime or Tyrion dies, which will leave the other sad in the end. If Jaime dies, Brienne will be broken. I think Grey Worm may make it, but is sad because he has no Missandei. Sansa is ruling the north alone, Arya won't stay (if she makes it - I'm not sure she will, given her conversation w/the Hound). I'm a bit bummed that there's presumably no more Tormund, that's bittersweet for me.