I can see Melee characters being added "officially", and I don't really see why not, if they're good and balanced and true to their originals and all. Especially once we get model hacking and that clone engine.
As for other characters--such as the awesomesauce Wisp--sure they may not be added officially, but I think there's a middle ground between that and only-for-home-play/maybe up to the TOs. And that is, something like a complex/picky approval process to form a roster of recommended characters. So they're not standard or official, but... they're still readily available for tourney. Kinda like suggestions. Or the stages in the Counterpick/Banned category--could be allowed, could be banned, up to the TO. But they're still there, still known, still options.
You know what I mean? XD I just think custom characters are too fun/cool/etc., and their creators put too much work into them, for them to just be silly little things you can play with for fun. If they're really balanced, and people really like to play with them, then why not allow them in tourney, at least in SOME capacity?