First off.. i know this won't happen. The Wii is selling just fine, Nintendo doesn't need to pull a rabbit out of their hat to impress people.
But imagine if they did. 3 of the biggest complaints against the Wii would be destroyed, instantly. Too kiddie oriented? Not with the nerdiest game ever coming out for the Wii and the wii only. Crappy online play? Not with an experience that puts the PS3 and 360 to shame. Not in depth enough to stay interesting? Haha, not anymore, not with a game people will spend days straight playing.
Once again, i know it won't happen, but it would actually make some sense. It'd atract a whole new group of people to the Wii, and monthly fees would ease the cost of running an online game like that. Nintendo has money to burn at this point, it'd be a great time to experiment with something like this. They wont, they'll play it safe, but it really would be cool if they did something like that.