I have a couple speculations that I think either are telegraphed or might be interesting.
1. The Night King isn't just going to go into a straight battle with the north. He has a dragon and is going to attack King's Landing first with it. This will happen after the golden company has arrived and basically end the Lannister's. Cersei will be turned into a white walker (The Night Queen?) and this new portion of the dead army will march north to pin and surround Jon and Dany's armies. (This is the last big Shock in the show)
2. Dany will not fight in the final battle due to being too pregnant. If they can screw with the time some she might be giving birth during the final battle (as a tropy juxtaposition). Jon will ride one of the dragons (having learned of his heritage) into battle.
3. Jon will use his dragon to fight and distract the night king as the second dragon lays waste to the undead army. This will work great until one of the giants kills the second dragon (somehow).
4. Bronn will use a scorpion with a dragon glass bolt to attempt to take down the night kings dragon. He will miss and hit Jon's dragon. It will not kill it but will bring it down and force Jon off of it.
5. The Night King will land in front of Jon atop Viserion. Jon will turn to face him only to see the frost dragon about to breath fire on him. There will be a dramatic slow scene where you see Jon's despair, possibly Dany is watching from far away, sad music plays, blah, blah, blah. Viserion roasts Jon with his frost laser beam thing. Everyone thinks its the end...but wait, could it be?!?! Jon, the song of ice and fire, literally walks out of frost fire, armor partially burnt away and beheads Viserion. The Night King, still emotionless, dismounts and fights Jon. Jon wins because he is a badass.
6. The hound will kill the mountain by using a fire blade, Jaime will kill Night Queen Cersei, Arya will find some way to die or be completely useless, Bron will die saving someone, Brienne will fuck the wildling guy before he or both die in the final battle, bran is going to watch things, Sansa is going to continue to do nothing, and Theon is going to kill Euron after he finds out his sister is dead. Note: Not all of these happen in the final battle.