Customizable moveset, if they don't become the new Fighting Team.
That, and the announcer was actually asked to do a fun announcement of the characters that people wanted.
Among them were Mii, Ridley, Krystal, and MegaMan, notably.
I think that was fan made, though.
Anyway...
Regarding the whole "Miis should be playable" and the moveset thing... Some of you might not like these examples, but... ever heard of Capcom Vs SNK 2's groove system? I say adapt a similar mechanic.
Let's look at body types. We've got skinny, normal, and fat.
Now let's look at height types. We've got short, normal, and tall.
Now, with these six combinations in mind, we pick one from each group and combine them to get a varied amount of movesets that would fit the specific Mii in question. Short and skinny? You get Mario, Kirby, Y/TLink, Olimar as possible moveset choices... Fat and tall? Bowser, Charizard, Donkey Kong. Just plain normal in both accounts? Link, Falcon, Luigi...
Or, if you feel like that wouldn't work, I'd like to imply a star system, reminiscent of Sonic Battle, having our Mii being the equivalent of Emerl. As you unlock a character, however, you unlock all of his moves (Let's face it, it'd probably be boring if you could only get a move or two unlocked per battle). With this star system, you have a set fighting style for your Mii that can be balanced (seeing as Mii-moves would be nerfed or buffed according to the original move that is emulated).
These are just ideas, and I'm not really with the whole idea of Miis being fighters, but if it happens, might as well be a well implemented thought out system that, hopefully, won't be abused (unless hacked).
Off-the-record, should the Mii be Wi-Fi available or not? If he does, there should be specific ranks (organized by fighting skill OR by Mii star ratings).
