What are his plans? Is he good or bad? Did he have any relationships with Ford? Who was the body he inhabited before Dipper's?

Everyone knows how Alex Hirsch takes your theories and just...they just get destroyed. (Except the Stan Twin one. :-D we were right!) So, does that mean he's going to do the same with Cipher being the antagonist? Probably not. Bill has had MAJOR foreshadowing on his oncoming presence.
'I've got big things coming. You can't stop me!'- Sock opera.
This is a big one. So is this...
'When I rise from my nightmare realm and unleash a...Well I'm getting ahead of myself.'- DAMGTMANF.

He will most likely be the villain. The reason being Alex didn't have the Deleted Scenes from Dreamscapers added because he seemed too friendly.

My idea of Ford's and Bill's relationship isn't that great. When Stanley came to Ford's house, Ford checked his brother's eyes and said 'I thought you were...*eyes dart around the room*...never mind.'
In Sock Opera, we saw a major difference between Dipper and Bipper. The eyes. However, could anyone else see them. I mean Mabel being his sister would've noticed the inhuman eyes Bipper had. My point? It was in Dipper's POV the whole episode. If it wasn't we wouldn't have seen Dipper in the first place.

So, Bill possessed Ford when he was a young researcher. That is why he was paranoid beyond his wits and sleep deprived.

So, was Ford Bill's first victim? And how is Bill going to get in the real world, because he can't function out of the mindscape without a vessel.
Thanks for reading these thoughts. Please comment if you think it could've happened or your thoughts in general.
Thanks again! :-D