Though i'm betting he is trying to pull a new Wii Sports/Nintendogs.
Actually, this makes absolutely perfect sense. Not just because of what Ian said about him being old and into that kind of stuff, but also because if you think about it the Wii U is going to need some epic casual title to launch with it, and probably be included with it as a bundle just like Wii Sports on the Wii. Look at how well the Wii did thanks to Wii Sports being bundled with it as a way to demonstrate it to everyone.
I'm with you and Ian that I want to see a new character adventure game, but I don't think that's going to happen if nothing else than because right now Nintendo needs to repeat the success of Wii Sports and pump out something in the same vein as that to be the casual killer app that sells the system to nongamers. If Miyamoto was the one behind the original Wii Sports, then its likely he is involved in whatever this mystery bundled game for the Wii U is as well.
The only thing is, will it be a collection of sports tech demos like Wii Sports? Or will it be assorted mini games like Wii Play? Something else? Or some combination thereof? It has to be something which has near universal appeal for it to be in line with Nintendo's strategy of targeting all demographics. This immediately rules a character adventure out, because these sorts of games have no hope of winning over nongamers. Sadly, therefore the only characters you will find in this game will be the bland personality-less Miis which you design yourself.