So here’s my theory, and it’s a bit out there: the Night King isn’t the “big bad”, Cersei is. I think this has happened before. We know the white walkers and humans have fought in the past, we don’t know how that ended. Humans are still around, so presumably they won, but maybe they didn’t. Maybe they made a pact of some sort, maybe humans eventually forgot the terms of that pact and violated it, which is why the Night King is back.
I think after a long battle, Jon Snow and the Night King will make a new pact. Then, when it looks like everything is good, Cersei will attack. We’ll get the remnants of Danaerys and Jon Snow’s armies, plus the remnants of the army of the dead versus Cersei and Euron’s forces.
I’m likely wrong, but hey, if I’m right, I’ll look like a genius, so fuck it.