This project seems to have been in the works for over a year now. I wonder if something about it is going to be revealed to us on the 24th? Maybe not, but it's fun to speculate anyway. Here is what we know about it according to the Wiki article...
Nintendo have greenlit a project at the studio within the Idabashi area that according to the official website describes it as, "The thoughts of game designer Masahiro Sakurai and Nintendo have become one, and a new project has started. In order to expand what can be offered to the world through games, we will have Nintendo's full cooperation and make something that Nintendo could not do on its own."
According to a set of "Iwata Asks" sessions on the Japanese Official Nintendo website, in the discussion, Iwata refers to the game as "A new experiment, both for Mr. Sakurai and for Nintendo." While admitting that he cannot disclose the content of the game, Sakurai adds "We've started a special project. We're not sure how it will sell or if it will be accepted. Our goal is not just to make a single game. We can promise that you'll be able to experience something different."
The new project, Iwata explains, came about as he told Sakurai that Nintendo was looking to make something new and asked if he would be interested. Sakurai explains that he took the offer because, as a freelance developer, the conditions were perfect—he'd be given a team, a company, and an office and left with the task of making a special project.
Iwata has apparently been privy to a glimpse at the game's design document. "I'm scared to ask, but how is the content of the new plan?" asks Sakurai. Iwata's response is: "From our perspective, it's not something that overlaps oddly with something that would come from Nintendo or those close to Nintendo. He later describes the project as, "It's something that can be made because Nintendo is backing you up, but is also something that Nintendo could not make."
The new game may have leanings towards core players. "If one were simply considering effective use of funding," explains Sakurai, "we'd have went with a plan similar to Touch Generations. But that's not I wanted." He later adds, "We want to make something that will be wanted even by the more hardcore game players, and something that offers new ideas."
I highlighted the bits that jumped out at me. These bits to me made this sound like it was actually going to be something related to the Vitality Sensor, but then there's this bit so I'm not so sure - although it doesn't necessarily rule that out.
Does anyone have any ideas what this could be?