It's been two years since their last release so I'm sure they're working on something that will be revealed at E3 most likely. A new Mario Strikers seems like a real possibility, especially since it would go with Nintendo's statement about releasing more Mario and franchise games on the Wii U, and this would be a lower-budget game. I wouldn't be surprised if they're making some other Mario spin-off either. They could also be working on another series revival similar to Punch-Out!! and Luigi's Mansion, though for some reason I find that less likely.