I suppose Miis could be edited for different types of games...two problems, though:
1. body types would all be about the same (only fatter/thinner and taller/shorter)
2. facial features would be a little harder...you could try to come up with something, but since (to my understanding) people can make their own textures for the faces, you might be forced to just go with whatever textures are programmed into the Mii. For example, if I can draw a bum and use that as the mouth for my Wii, then how would a facial-feature recognition engine interpret that? Although it does look like there are height and widht limits on the features, so maybe it wouldn't be so hard.