My Undertale AUs

TheoryTale

It starts with science sans in his original au. There are some other characters in this one, in the lab, who influence him and talk about their theories with him so he gets them all in his head to firm one ultra theory: That the vessels for the seven Monster Aspects (reflect of the seven fallen children aspects) are the same as the monsters majorly interacting with frisk in the Omega timeline, tying in with gaster's multiverse theory. Nobody beleived him so he researches on his own with nobody to help him while managing time with his baby brother papyrus who loves him whole heartedly and tries to help even though he can't actually help him all that much. It's adorable anyways.
They grow up and Gaster disappears during an experiment with DETERMINATION leaving Alphys in charge of the laboratories. Alphys, who doesn't beleive in the multiverse theory or anything surrounding that, shifts the focus of the lab strictly on DETERMINATION and trying to bring gaster back from, presumably the dead, with some genetic samples left around. Sans, with all his information and research, beleives if he finds somehow an aspect for gaster, like a hidden aspect, he could bring him back. He thinks it not a matter of determination, but a matter of SOULS. Of course, nobody believes him except his little brother papyrus. Papyrus helps him in his spare time but ultimately prefers training or patrolling for humans.
More on the topic of papyrs, he has grown a lot less happy go lucky and he doesn't tell as much. He's more worry some but stays optimistic. He tries to look for the bright side and often has to remind Sans to take care of himself or some out if his research she'd next to the house. It left Papyrus with Sans's original depression from the Omega timeline and others a lot like it. It made him have to learn to do things on his own.
Sans actually managed to find a way to bring doctor gaster back. But by this time Alphys already experimented with the amalgamates and shut down Hotland and lab so she can store them there. Only one lab remains and that's the snowdin lab. Of course, nobody beleives Sans's claim that he can bring gaster back. Sans had captured flowey and forced him to fuse with him, allowing sans to get the power to reset. He uses this power to reset and he uses the manifestation of his own soul to open a portal to the void and pry gaster out. Gaster was revived yet at what cost? Sans dusted himself to bring his father back. The saddest part is, Papyrus was there and he witnessed the whole thing.
Papyrus is thrown into a deep despair. The shattered fragments of his soul remain on the table. Papyrus picks them up and absorbed them into himself. He doesn't change physically but now mentally he has sans with him like an imaginary friend. Similar to Papyrus to sans I'm asylumtale.
Gaster is back and he is informed everything that has happened while he was away. He is now cracked and deformed due to the experience of being trapped in the void.
Papyrus, since he literally has sans in his mind, tells him about Sans's research and experiments. But no matter what the all been through, gaster still refuses to test it staying it was impossible. Papyrus gets mad and emotional, after going through a whole lot and all.
Now, let's look at the power papyrus has. Papyrus has flowey's ability to reset and change fate, his own normal optimism and enhanced power by training, and Sans's smarts, logic, and power. Papyrus is basically a god at this point. All he needs is a human soul and he will be the most powerful being Ever.
In so much rage, papyrus literally opens a rift into the void and leaves, not even knowing where it can or will go.
That leaves papyrus to spend the rest of his days with his brother forever in his concious, traveling from AU to AU desperately searching for proof that his brother's theory exists because he loves sans so much.

He will visit AfterTale, then UnderFell, then Underswap, then Undertale (Omega universe) until he is found as an anomaly to Error and taken by force to become one of his puppets to be hung on display.