I've been seeing a lot of works for the AU where Stan falls into the portal instead of Ford, and they're great! I understand having the face of Bill's biggest target wouldn't make things easy, and it was probably still traumatic, but consider an alternative:
Stanley Pines would thrive in the multiverse.
At their cores, Stan and Ford are two wildly different people, and thus would have a vastly different array of experiences when put into the same (albeit constantly fluctuating and inherently weird) set of circumstances. Ford's reaction was to keep a low profile, and detach himself from others (the Lone Hero complex he struggles with through the series).
But imagine Stan, thrown out of the portal and alone with nothing but the clothes on his back, a situation he is (painfully) familiar with, only now he has ten years of experience with the situation and knows exactly what needs to get done to ensure his survival.
Stan, dropping seamlessly into his salesman persona, charming the pants off of a band of rogues in the nightmare dimension, and they show him how to use the portals to get around the multiverse.
Stan, who's spent every night since he was seventeen in his car half a degree from freezing to death, being thankful for the temperate climate of the first world he falls into. He still worries about Ford, but he sleeps better his first night in the multiverse than he has in a long time because he doesnt need to set alarms every ten minutes to ensure he'll wake up again.
Stan, who (despite the looming threat of Bill over his head) can finally breathe because there's nowhere in the multiverse he's banned from (yet), no cartels out for his blood, no constant fear of human enemies because he's the only human for lightyears and he feels ten years of tension flow out of his shoulders.
Stan, who starts picking up knickknacks in various worlds and surprising himself when the cool rock he found on the desert planet sells for an astonishing amount to those mantis people. He taps into his haggling skills and walks away with more money than he ever had on Earth.
Stan, who buys a huge knapsack to keep odds and ends in, and charms peddlers into revealing the locations of valuable merchandise across the cosmos. Sometimes he brings them along for a cut of the spoils, other times he regales them with tales of daring escapes and brutal battles. He collects scars along with his wares, but all of them bring a spark of old excitement into once-empty eyes (save the symbol on his shoulder, but denizens of the multiverse learn not to ask).
Stan, who turns the cons of his childhood into respected business practices. He makes enemies (you can't change nature), but leaves behind a trail of people he's helped in his wake. The interstellar police knock on the door of an entity made of pure energy. The entity politely says it can't help them. When the police are gone, Stan pops out from behind the couch, congratulating it on a great performance.
Stan, behind whom grow tales of the wandering nomad with quick hands and a quicker wit, who has contacts spread across the width of the galaxy, who could be the bringer of salvation or destruction (Stan laughs when he hears that, but the slime child notices the pride quirking up the side of his mouth).
Stan, whose first instinct isn't to hide, but to connect, because he's good at it.
Stan, who has the adventures he dreamed of as a child, finding treasures beyond belief (and a good number of babes along the way).
Stan, who thinks of the portal back home as just another of the same, and the first time he re-enters into 46'/ he reels back and delivers his signature left hook straight into his brother's face. It takes some convincing for Stan to realize he didn't just punch a shapeshifter (because it never once occurred to him that Ford would work tirelessly for years to get him back the same way Stan would for Ford).
