I know everyone has played Undertale but that would be a major spoiler (another reason why I don't think Sans is a choice).
There's mad moveset potential for that form of Asriel, sure, but it's also like...I associate that version of Asriel with, gameplay wise, choosing NOT to fight, and it would be a tad ironic for him to be in a fighting game.
Also I always cry during that boss fight and I don't wanna cry during smash
I guess the complicating factor for Undertale is that like, the whole point of the game is about the virtues of making friends with your enemies instead of fighting them. Undyne is the only character who likes fighting with her friends.
It's different than Villager and Isabelle, bc Animal Crossing isn't about why fighting is bad.
Yes, Toby knows all of Susie's story, but we haven't seen it, and would he really want to spoil a character's arc in Smash?
EDIT: needed to fix a few spelling mistakes