I’ve honestly stayed partial to the Snoke = Plagueis theory since 2015, and Palpatine coming back does little to change my mind LOL

Plagueis found a way to cheat death like he wanted, finds/is found by the remnants of his former apprentice’s Empire, and assumes control of it, thinking himself obviously more powerful than his apprentice. But Palpatine from the grave somehow influences Ben to join the dark side and help Snoke, under the guise of being the whispers he hears of his grandfather. In TLJ, Ben, all according to Palps’ plan, betrayed Plagueis/Snoke, re-asserting Sidious’s dominance over his master.

With Ben’s faith in Vader restored, maybe whatever macguffin everyone is after in IX - Ben will have thought he was pursuing for himself/Vader - but will be revealed as what whatever Palps’ ghost wanted all along.