The WiiSpeak for voice chat adds a social dimension that Sony and Microsoft have had for a long time and Nintendo sorely lacked. What would be really fun would be the ability to attach "catchphrases" to your Mii, so that your voice and favorite sayings can be added to the avatar that represents you. That would be really fun for Miis mingling between Wiis. And it would be really sweet if you were playing Wii sports, and when your Mii scores or something it could say "Woo, allright!!" the same way you do when you yourself play a game.

Ideally Mii catchphrases would have "moods" too so that they say the appropriate things when good or bad events happen in games.