Spoiler:
I think all will die except maybe Sansa and Arya. Doesn't matter who kills who, because...
...we shouldn't forget about the one person whom I think is most important:
Spoiler:
Bran. The reason why I think so is because there has to be a reason why the end of Season 4 and most of Season 6 was so focused on him, the Three-Eyed Raven and the ancient lore of the First Men/Children of the forest, which hasn't been addressed nor concluded in season 7 or 8. He knows the past and present and told no one about Cersei's plans nor about the foreshadowing of Danaerys' mad turn, or Euron's fleet etc. because he's just sitting back while the humans finish each other off. Because Bran is no longer 'Bran', I think it will go full circle to what started the song of Ice and Fire in the first place. The Three-Eyed Raven helped aid all the humans in defeating the Night King but let all these other shitty things happen, so that they would be no more hierarchy, houses, or 'modern' civilization with the burning of King's Landing.
With the Night King gone, and most of the humans/royalty gone to claim land or establish or continue new houses, the Children of the Forest get their peace and land back, which is why the Night King was created in the first place, which is to take what is rightfully theirs, and now as everything is crumbling, they've waited years to get to this point and finally will have it.
So, if this happens, I would be very satisfied overall with the series despite the garbage writing. Also, I remember GRRM mentioning he spoonfed D&D the ending of ASOIAF and even though they've deviated from the books, it all has to come down to the ending he wanted which will reflect the books apparently. I hope it is along the lines of the above.