Enemies from the characters' origin games! It would be really cool if they did a Subspace Emissary-type mode, but then it involved you travelling in areas from each series represented in the game, fighting the enemies from that series. Maybe you'd start as Mario and Luigi in the Mushroom Kingdom, save them from something, and then make your way to Hyrule to do something there, having characters join you in each world. Maybe some enemy is trying to do something to all of the worlds or something, so you need to travel to them all and save every series' world! Could be a fun way to do a sequel to SSE.
Would you rather have Mii Fighters or custom moves return?