I would love a full-on Mii game with a storyline. These characters have become very iconic, even though they're usually generic avatars or background elements. An RPG is not a bad idea for the genre, but they have to be careful with the execution. There are many modern series that take on the "generic" traditional RPG, like Denpa Ningen. What would make a Mii RPG special?
Incidentally, I've always wondered what it would be like to explore Wuhu Island more freely. There's a cycling game in Wii Fit Plus that kind of touches the idea, and it's certainly nice. Expanding to more locations and adding Mii NPCs and a story would make it a very pleasant game.