I know I'm a broken record on this, but I really hope they don't add Sans. It would be one thing if his moveset was like Villager's, where he fights by not fighting. It wouldn't be great, but I would be alright with it. But there's no way they would do that. If they add him, he's going to be based around the Genocide fight, and that's what I really don't like. Making the crown jewel of Undertale representation in Smash completely based on a boss fight you can only get in what is essentially the bad ending. It's like if the playable Three Houses character in Smash was specifically
this version of Edelgard. I don't want to sound like some sort of prude or something, I don't mind the mii costume, but I really hope the actual playable Undertale character represents more of the game than an (admittedly very cool) secret boss fight.
I agree that an Undertale character is inevitable, but I really hope it's someone other than the Bad Time man. Papyrus, Toriel, Ralsei, Susie, take your pick.