It could be a massive misdirection. The Night King might attack King's Landing directly with Viserion? Do they have anything at KL that could combat either one of them? They don't even have valerian steel weapons there anymore?
Edit: omg, "King's Landing". It became a real king's landing when Tommen met the ground, but it could also refer to the Night King landing there and laying waste to it. That's exactly the kind of wordplay that GRRM has been practising.
I kinda like this angle. It is probably incorrect, since they set up the battle at Winterfell so clearly this episode... but it would make for a fantastic turn of events. In Dany's vision, didn't see we the throneroom covered in snow and without a roof? Dany would be ruling over the ashes of Kings landing.
A lot of people are mentioning the idea of the corpses in the crypt being resurrected by the Night-king. This seems a bit too predictable, plus I would expect the old Stark magic to ward those crypts from any such foul magical tempering. But right now its anyone's guess.
As for predictions of who will live and who will die, any guess will probably be wrong, but here goes:
Survivors:
The Hound - Might survive, so he can face off against his brother. I'm still holding my fingers crossed for this.
Jamie - Not only is he my favorite character, but I feel we still need a confrontation between him and Cersei.
Arya - I think she still has an important part to play in the finale as an assassin.
Dany - What would have been the point of the vision of the throneroom, if not for her actually going there?
Samwell - I keep thinking that he is the person writing all of this down for posterity.
Dead meat:
Beric Dondaryon - The lightning lord's death is long overdue
Tormund - Sorry tormund fans.
Gilly - Not very useful, and makes for a dramatic death.
Brienne - I think it is time for her to die heroically.
Theon - No one cares about you Theon.
Jorah Mormont - He will definitely die for his Kaleesi.
Lyanna Mormont - GRR Martin will definitely kill children.
Davos - Likable, but ultimately not that important.
Ed - Sorry Ed.
Missandei - Bye!
Grey Worm - Also bye!
Ghost - You are costing us way too much of our CGI budget! We gave you a quick cameo in episode 2 just before we kill you off unceremoniously.
Unsure:
Jon Snow - GRR Martin has a habit of killing off characters that seem to be the lead. I don't think a happy end is in store between him and Dany, but he might still survive this battle.
Sansa - She's gone through considerable character growth, but I am still unsure what her role is in all this.
Melisandre - I think she still might have some role to play in all this, but not sure what it is.
Bran - He may very well live... for now.
Tyrion - He feels like the most important character in the whole show. But GRR Martin loves to defy expectations. His usefulness seems to have decreased lately. But episode 2 suggests that he might make some sort of clever come back.
Not mentioned:
Anyone not mentioned probably does not appear in the upcoming episode, and thus automatically survives.