I will list a couple of things that would be cool to do with the Miis (Ideas)
A channel where you can connect to a virtual city full of Miis (made by people) and interact with them. The best example of this is Habbo hotel (a virtual community made in flash) Imagine going with your Mii. That would be a good heavy used channel that will make people use the Wii more often (great for non gamers too). it could include a few minigames as an extra interaction between users.
2)Online funcionality for Wii sports and Wii play: This explains itself already
3)A Mii channel update that would update the Mii editing making the player able to change aspects of his, her Mii's body like clothes and shoes. (also fuly editable so you can make your original clothes and use them or even sell them on the Mii Nation. This would start an economy system on Mii nation) Original clothes designers that would be cool.
4)A Mii Olympics games (with online play)
5)Mii pseudogames like a simple online survival horror, rpg etc. (with the complete editable characters it would be a blast) it could be named like "Wii action online" lol
These are my ideas, plz share yours