Although using the Mii in Smash Brothers seems to have a real appeal to me, I belive it would end up being very limited, Would every Mii have the same moves? Because it would be too difficult to implement (and balance) different Mii's having differnt moves. If it does get implemented, I would like to see a single "Mii move set" and all Mii's would use it, and just their appernce would be differnt.
But to the point I think a really neat alterative to using the Mii would be to have each character more customizable, most characters in SSBM can change colors and some can change suits. But say, for example, if you could unlock a new sword to choose from for Link (apperance-wise only). Or possibally the choice to make mario appear as Super Mario. (Ofcoarse that is only assuming SSMB will have no move already assoicated with marios cape.
These are just ideas and i want to stress that the customizability would have no effect on anything physical in the game. I would see this feature being useful in the situation that somone else was picking a character that I enjoyed to play as.