A theory I've thought about. We've seen that spiral symbol the army of the dead have left behind since the pilot episode. Also saw it in season 6 when Bran sees the Night King's "birth." There was pattern of stones the same way in the pattern where the corpses were laid out in the pilot episode, and then in season 8 episode 1 with the body parts pinned to the wall.
Some think this is just "their symbol" and it won't have importance, but I think it will. Chronologically to the story, the first time it's revealed is season 6. I'm guessing it's some ritualistic symbol of the Children of the Forest. So why does The Night King/army of the dead keep leaving this behind? I'm guessing it's a hint to their goal, the one major reveal we've yet to discover. My feeling is The Night King wants to return to ground zero of when he was turned, where it all began. Maybe there's something there that can break the spell or there is a settlement of Children still living there that he can wipe out once and for all.
I even read one theory that the place he was turned(theorized to be the God's Eye) may be the main source of magic for the realm. That all the magic in the show, the "Lord of Light" resurrecting people, the Weirdwood trees supplying power, the White Walkers and their magic, etc. It all flows back to that one spot. And maybe the Night King wants to go there and destroy the magic source of the realm to either turn them all back to humans, or to prevent anything wicked from happening again in the future, and end the use of magic for all eternity.
I'm hoping it'll be something elaborate and they don't just turn the Night King into some cookie cutter villain who wants to kill everyone, that would be so lame.