Everyone knows about the theory that Bill Cipher is still alive and kicking, but that creates lots of other questions and paradoxes.
Here's my theory, Stan obv got all his memories back, with those memories, Bill came back too. But, if this is true, it's either he randomly respawned in some random area on earth (I'm not ready to accept that ALEX HIRSCH of anyone make it that simple), or maybe he is still inside Stanley's head. Suppose this is true, after he handed the Shack to Soos, he left to go on adventures with his brother, Stanford. Where were they headed to first?, well the show is kind enough to tell us that Stanford had detected some unknown anomaly near the Arctic, hmmm, strange to think that an anomaly would just randomly appear out of nowhere. Unless, Bill somehow spawned it or maybe caused it to appear, I don't know if he would do this alone or with some help, since he's trapped in his dimension by the end of the show, but he did it to isolate Stanford and take the secret equation to get by Gravity Falls' Natural Law of Weirdness Magnetism. So, both the brothers are on the "Stan 'o War II", Stanley in his possessed state asks him the equation, the unaware Stanford tells him. With no more need of Stanford, he drowns him in the sea, with Stanford out of the way and the journals burnt to ashes, no one is able to stop him, he tricks another fool to let him out of the mindscape and into their dimension, starts Weirdmaggedon 2.0 and takes over the universe.
Yeah, he's basically unkillable at this point lol. Anyway tell me what you think.