I think it'd be awesome to have Mii's in brawl. Here's the process I think would fit:
-Choose a Mii from your Wii's hard drive
-Assign attacks for every B and A move (from a list of attacks, possibly attacks of the characters already in Brawl)
-Adjust sensitivity to damage
-Adjust jump height
Heh, Imagine Chuck Norris vs. Mr. T in an all out brawl.