Shigeru Miyamoto has stated in a recent interview that he's very interested in the notion of user-generated content in Nintendo games, which has given rise to the hope that he's prepping some kind of LittleBigPlanet-style title.
Speaking to Joystiq, he had this to say about the concept:
This is an area that I have a lot of interest in, and I think that the side-scrolling Mario games in particular are well suited to that idea.
Of course the Mario vs. Donkey Kong series is one that’s made in North America by NST, but it’s one that I’ve been involved in and from early on. We did have a lot of discussions about how to enable that kind of level creation in that series. And I think those types of interactive experiences are going to continue to broaden and we’ll see a lot more user-generated content going forward. I’ve always had an interest in those kinds of creative tools, so I will definitely be open to ideas like that in the future.
Just imagine a game in which you could create your own world and place Mario in it...sounds great, don't you think?