Some potential leaks to munch on:
I dunno, this feels legit to me. It makes a lot of sense with what we know so far - Makuta just vanishes after the battle, unlike Ekimu, so being trapped in a ghost world of some sort definitely feels like an route they could have taken as to his fate. He needs a mask (or masks, my guess is the three primary ones) to reanimate himself, so then he uses his ghost powers to start reanimating boneheads to grab masks en masse for him. When that fails, he pulls out the big guns and summons his ghosty friends to destroy the Toa and claim the mask of Control so he stands a chance against his revived brother and his mask of Creation.
If it is legit, Mask Hunter is 100% Makuta.
e: wait no thats dumb, it'd have to be Ekimu in his new form (made to find masks before Makuta vs. making masks as he originally did), since that's a consistent part of the leaks. Plus that means Makuta doesn't get needlessly switched out from "Mask Hoarder"
IMPORTANT EDIT: Calling the 3-year arc's endgame right now: Doo Head face shape of the Ancient City means there is a giant robot under it. Since Makuta is a ghost he is going to assume control of the body. This year he's trying to get grinder to boot up the robot via Mask of Creation in the forge, so he can assume control. if they continue doing a new mask every year, then in year 3 he will succeed using the mask of ultimate power somehow.