I posted these ideas in the Neoseeker forums.
-Customizable moves. For example, if you are customizing Fox, you can choose to give him moves from either his movelist, Falco's, or Wolf's. Yes, I do think that certain "clones" should share slots with base characters by being costumes for said characters, but still have their movelist available.
-Custom colors
-The ability to change stages from being dynamic and hazardous to static and balanced. An example: Warioware in Brawl is what I would consider a "dynamic" stage. It would be cool if you could make the stage "static". By doing so, the stage wouldn't change into the minigames; it would remain as the starting platforms for the entire match. The point of this is obviously to expand the list of tourney stages.
-Playable bosses.
-Percent ceiling mode, where a match plays out regularly. The only difference would be that you could be KO'd when your percentage reaches a certain amount. The amount could either vary from character to character or be determined before a match.
-Tag battle. In Subspace, you could select multiple characters. When one character was KO'd, the other would swap in. It would be cool if this feature was available for regular matches. I understand that this concept has already been used for PokeTrainer, but tag-team combos would be pretty interesting.