Reunion Falls AU - a gravity falls au in which mabel and dipper, for (possibly supernatural) reasons unknown, were separated at birth, and while mabel grew up as an only child in her normal home with normal parents, dipper grew up in gravity falls raised by grunkle stan. but at age 12 mabel learns of dipper’s existence and decides to pay her long lost twin a surprise visit, completely unaware of the kind of wacky business going on in his hometown more on this later!!
If they ever start a new series of Gravity Falls, this is how I want them to start it: A kid goes to shake the hand of the statue, but he's stopped by an old man. He's warned not to, but once the man leaves, he does. Then, the series slowly builds until finally, the old man (Grunkle Ford, Grunkle Stan may have passed by now, since I think that the whole portal travel thing aged him differently,) has to call in an older Michael and Mabel Pines, who give the new character remade Journals with…
Grunkle Stan: KIDS!!! KIDS HE'S BACK!!! Dipper: W-wa. Grunkle Ford: THE DEMON WITH 1 EYE!!!!DON'T COME TO THE TOWN!!!!! Grunkle Stan: WE DON'T WANNA LOSE YOU 2!!!!! Mabel: We are on oure way. Grunkle Stan: NO MABEL!!!! *Silence* Dipper: Lets go.
Edit: This post was made before we knew for sure that Stanley/Stanford was the author, or that he even existed. As such, this theory does not line up with actual events in Gravity Falls. It is super cool though!