Maybe the Excite Truck sequel will get revenge and copy Motorstorm this time and take out time trial and offline multiplayer. That'd take a lot of time.
If it's so secretive, maybe that means it's not a racing game? I hope it is though. There's also rumors that Firebrand are making a (or two) Wii game, makers of Create & Race and Trackmania DS.
I don't think it's F-Zero because that'd be the kind of project they'd be joint developing with Nintendo and it sounds like they're all sitting in their offices by themselves. Excite Truck 2 or a new project is my guess, can't wait.