We're probably all tired of theory after theory by now, but I just thought I might as well post one:
My theory is that Bill is a combination of everybody in the wheel, and his ultimate goal in life (or whatever you would call his existence) is to travel back in time and get his past selves to become him. He does this by getting them to shake hands with him somehow, (so he now has Dipper and Gideon).
In Dreamscaperers he is able to take Soos' form and voice and impersonate him perfectly. Also, when he says "I'll be watching you", if my theory is right, then he isn't watching them at all, but simply has all of their memories, and so doesn't need to.
In Sock Opera he says "It's been so long since I've inhabited a body!" perhaps hinting at this. I also think that his motive for destroying the journal was to stop Dipper from figuring out who exactly he is.
I've heard people say that their is a variant of that theory, so at least I am not completely insane. Post your thoughts!