Miyamoto said in an interview with Zoomin Games that he's begun work on a brand new, original title. Whether that means a new version of an existing franchise or something entirely fresh is unclear. Check out the interview above and draw your own conclusions—Miyamoto also responds to last month's false alarm regarding his retirement.
They also mentioned that he's working on Luigi's Mansion 2 and Pikmin 3, a title we haven't heard much about in a while.
We're guessing the new title will be on the Wii U, so hopefully we'll hear something about it at E3 in June. Let us know if you have any theories in the comments or on Twitter.