I think it's fine in any type of game...as long as they're actually not developed in time to be included in the original release. The closest thing in games that I play is civs in RTS games. A civ in an RTS is basically a "character" in a fighting game. New characters in a MOBA would probably be the same. And turn-based strategy civs.

These all require extra dev time to implement new mechanical features. Extra balance work to make sure they're well-balanced (both on release, and later with future balance patches). Extra design work to make sure they look good and unique. It's completely reasonable to charge for them. Just not if they're there, ready to go, day one.