I just had a stupid idea that'll probably never be added.
Combiners. "But PhatBoy, combiners are too big for FTF!" and Ridley's too big for smash, bite me. Combiners could A: Have their own questline/story, and arena fights with combiners or anything like that, B: be a special 4 for any member of the team after collecting the teammates, or C: Both. Having a combiner storyline would pretty fun if you ask me, though extremely unlikely. having them as a special 4 would be a cool way to incorporate them, but underwhelming, doing both would be sufficient and possibly bring in more players and increase the size of the fanbase, the game is already fun, and this is an idiotic suggestion, but I'd like to make it anyways.