ok so I've been in a pretty exhausted episode lately but I just want to say that I love the theories of Sans coming over to undertale from deltarune by means of time travel or something similar to that. this theory is also probably, very canon, and it would also be cool if it was confirmed to be the real deal....
... but in my humblr opinion, I'm a sucker for family tropes, and I love fan-canons that coin gaster as being a father to sans and papyrus. and a father -not out of reproduction with another monster- but out of making them with his own scientific prowess. growing them from inside two encased jars, the coldness of the underground laboratory. the realization of his first successful experiment where he finally created something that is alive and breathing. for his children to be raised in secret, away from other monsters in their childhood. a few years down the line, he lives near the grand castle, his son begins working with him as an assistant and one other co-worker. another experiment. things go horribly. he disappears, or falls into his own creation, rumors start to spread. a few years later, two skeleton brothers move into snowdin town, away from the commotion of the bustling kingdom.
like, holy shit, that would be cool as hell if it was canon...however, the truth comes to pass, and I genuinely think having this mystery surround Sans's origins is what makes him so interesting. he's not even a major character in deltarune (yet), but I really do hope we get some good stuff with him that isn't just courting toriel.