alright let me take a crack at this
there are a few plot holes that we need to avoid here. and by a few i mean a good amount.
stan’s criminal record. nobody would want to take the identity of someone who still has jail time to serve. so this will have to be an au where a. it takes place before stan turns to crime or b. stan doesn’t commit crime at all. or a secret third option where ford doesn’t even realize how much of a criminal stan is until he’s too far in and has to learn how to run from the cops.
the fingers. nobody in gravity falls ever noticed that ford had 6 fingers, so it was easy for them to just go with a normal guy who had 5. but someone with 5 fingers seeming to randomly grow an extra finger? strange. especially since stan is much more sociable than ford. the guy who’s known for being super talkative and knows everyone in town suddenly locks himself away in his house? also strange.
stanley needs to be out of the picture one way or another. probably not dead?
ford needs a motive. this has two parts two it. a. he needs a reason to leave his old identity behind and b. he needs a reason to take stan’s identity specifically.
so with all of this in mind. i give you this.
stan gets to gravity falls before ford does. thinks “hey this place isn’t on any maps. no cops will find me here!”
settles in. makes a good living off of being a tourist trap.
he uses the bill summoning thing as another attraction. Very Stupid Idea.
bill comes to make a deal with stan. thinks “hey look there’s a guy i can con.”
stan gets approached by bill for a deal. thinks “hey look there’s a guy i can con.”
stan goes missing
ford arrives to gravity falls.
ford does not have grant money. people don’t believe in his weirdness theory because they think he’s just talking about the tourist trap that used to be there for a while.
finds an old abandoned shack.
investigates old abandoned shack.
“hey wait a minute. not only are there scraps of proof for my theory, but these are stan’s belongings!”
ford thinks. nobody takes him seriously. people don’t seem to take stan’s disappearance seriously either.
“hey! what if i took on his identity, reopened the tourist trap, and used this footing for money and such while i look for stan?”
and thus. ford takes stan’s identity, runs the tourist trap and tries to look for his brother.
the excuse for the fingers is that he had a run in with some kind of magical creature. it gifted him with extra dexterity in the form of extra fingers. everyone loves it.
ford learns to run from the cops. something he never thought he would have to do.
You know what? We have Stanley pretending to be Stanford for a long ass time
Now give me an AU where Stanford pretends to be Stanley. I don't know man, like he wasn't anyone you could even pretend to be. Stanley is a nobody. Basically.
So this AU would be where he is someone somewhere and Stanford has to pretend to be him. Why? I don't know, figure it out.