Could be interesting to see what people come up with.
I figure if it's added though the problem would be that maps would require every player to have.
Cosmetics would have to be client side and picking that option would have to make it so it appears as like the default character to people who don't also have the mod. That or make it so a cosmetic can only replace one that already exists.
Will also be cursed when I see someone make the inevitable Pickle Rick character mod.