I know it was implied that Bill destroyed his dimension along with everyone in it, but I'm not convinced.
He specifically said that he "liberated" his own dimension. Yes I know that when he says that, we see the image of a 2D planet burning in his eye, and screams can be heard while it goes up in flames. HOWEVER, I looked up the definition of the word "liberation," and it quickly got me thinking that maybe I first took the meaning of that sentence too lightly.
Liberation (noun):
• set (someone) free from a situation, especially imprisonment or slavery, in which their liberty is severely restricted
• release (someone) from a state or situation that limits freedom of thought or behavior
Yes, I know that in the show Bill seems to think that his Weirdmageddon is somehow making "making [Ford's] dimension free," and that "anything will be possible" when he's done with it. Again, I know that the visuals onscreen are showing Bill committing detrimental actions, but why on earth would Bill show Ford those visuals? Bill already knows that Ford doesn't want him here in the first place. If anything, he's making his odds of getting that equation much, much worse.
What if those images -- of Bill carving his finger into the US, of him eating half the planet, et cetera -- were actually an imaginary sequence in Ford's mind? That he was picturing Bill would do horrible things if he were set free? I think it makes more sense than Bill showing them to Ford.
(Yes, I know we see the first and last pictures specifically in the bubble that's coming out of Bill's eye, but it just doesn't make any sense to me any other way. Ford could be hallucinating for all we know; the chain on his ankle could be affecting his brain's visual processing functions. Or maybe just the visuals in the middle of the sequence were from Ford's mind and the others came from Bill. I'm not totally sure.)
But more importantly, none of those things actually ever happen anyway. Sure, maybe Bill wanted to demolish earth, but he didn't. (I don't even understand why he wanted to destroy it; I mean, why spend so much time screwing up Gravity Falls only to bite it off of earth and move on to other planets without any life on them? Bill clearly enjoyed ruling over humans, so why would he want to kill them?)
What's SO great about this particular dimension? Ford makes it clear that there are hundreds of others in the multiverse. Why does Bill want this one so badly?
Aside from anything stated canonically on the matter, my theory is that Bill has no where else to go. He physically CANNOT rule any dimension besides Ford's or the nightmare realm. Why?
Because he's not powerful anywhere but the three dimensions where we've seen him reside before: his home dimension, dimension 46'\, and the nightmare realm.
It would explain why Bill never followed Ford to try and destroy him after he escaped the nightmare realm. It would explain why Bill knows Time Baby but never attempted to kill him before. It would explain why Bill wants 46'\ and no other dimension. It's because the nightmare realm will collapse soon, and he can't go back to his home dimension (either because he destroyed it, or more likely, that he just caused some kind of destruction and slipped out among the chaos).
So what does this mean? Well, mostly that Bill isn't a god, but merely has god-like powers in this particular dimension. Think about it: Ford could have been considered all-powerful in some of the dimensions he visited. (Like in the baby dimension, where he would be the only adult, and thus the only person with fully-developed motor skills.) Maybe back in his home dimension, Bill is just another immortal shape who can fly and manipulate reality, just like everyone else.
Bill wants 46'\ because everywhere else, he's just another nobody.
Bill has always seemed like a rowdy teenager with superpowers to me. Maybe that's because that's all he is. Maybe he didn't destroy his dimension, but instead he's just a renegade from it, who simply wanted to live in a dimension where he truly meant something:
a dimension he could be a god.