Someone’s probably already thought of this before somewhere, but I’ve never seen any content on it. So Dipper fails to save Ford in DaMvtF. The prison droid leaves the Earth’s atmosphere, flinging Ford off-planet once more.
Luckily, Dipper has the adhesive and manages to use it to… make a fairly sturdy patch job on the rift. He was never told exactly how to go about patching the cracks, but he figures it can’t be any more difficult than fixing a ceramic mug with super glue. In this AU, then, Bill Cipher would cease to be an immediate threat. The rift is enclosed and in a secure place, and unless Bill possessed Dipper himself it ain’t going anywhere since he and Ford were the only ones who knew about it at the time. And you can sure as hell bet Dipper ain’t making any deals with him.
Meanwhile, the prison droid is sending Ford to some prison complex light years away. I have this awful image in my head of the bubble thing he’s trapped in transforming into a cryogenic stasis pod once it leaves atmosphere, Ford thrashing about in panic as one of those cord things that grabbed him in the episode pricks him with a hypodermic needle that fills his veins with a special fluid that will keep his blood from freezing while in stasis. It’ll probably take hundreds of years for this small ship to actually reach the prison.
On Earth, Dipper rushes home in an absolute meltdown, babbling almost incoherently to Stan and Mabel about how Ford was caught by the droid. He fears that Stan hates him now because he couldn’t save Ford and he’s gone again JUST AFTER Stan spent all those years working to get him back. Stan could never be angry at the kid, though. He hugs him tightly, whispering how it’s not his fault and he tried the best he could. They begin to go over their options for how they might save him, which admittedly, are very few.
Eventually, Mabel mentions Blendin, and they end up doing something to contact him for help thousands of years away. (Hmm how to attract a time traveller…) In the book Dipper and Mabel and the Curse of the Time Pirate’s Treasure, it’s established that Blendin’s time tape can take him through both time AND space. So there’s how they’re gonna reach Ford, eventually. The only issue now is how to find him. Where was he actually sent? And how many years would it take for Ford to get there?
Hundreds of years into the future, Ford wakes up at his destination. But rather than wake in an operational prison facility, he finds himself wandering alone. The place is abandoned. Rusting. Falling apart. As if civilization as a whole on this planet has ceased to be altogether. It’s not a surprising conclusion to him- after all, the craft that collided into the Gravity Falls had been there for millions of years. Whatever alien civilization once existed here must have died long ago. HIS goal is to become acclimated to his current surroundings and the reality of being homeless among the stars again. He doubts rescue is possible at this point, because his family hasn’t been alive for hundreds of years.
But his family is coming for him. They’re crossing time and space just to save him, and nothing can stand in their way.