How about Underfreak, for the headcanons?
In this AU, magic has been corrupting, changing all of the monsters to look more monstrous. Asgore and Toriel became the scariest looking out of them all, so they hide away. Everyone acts just as kind as they normally do.
The more magic the monster has, the more monstrous it started to look.
The monsters are no longer trying to get out of the underground. They know that if humans see what they look like now, they would never accept them.
Monsters won't try to attack the player, and instead tries to avoid them. You can bump into them though while playing and either battle or help them calm down.
Sans still loves to make puns, and he's one of the monsters that would go out of his way to talk to you. The others being Monster Kid, Undyne, Papyrus, and a few others.
Papyrus doesn't realize how scary he looks, and is trying to catch humans to get out of the underground. Nobody cares enough to tell him what was going on.
Sans is Breaker and Papyrus is Undertaker