"We'll have Mario Kart 8 out next spring, and at that point we'll have the whole lineup in place, so I think it's about time we brought out a new franchise," Miyamoto said in the latest issue of Famitsu (as translated by Polygon). "I can't give out details, but that's the title that's been keeping me pretty busy lately."
A new and original game from Miyamoto was first revealed in January 2012, when the Associated Press reported that the designer was at work on an "undisclosed original title."
He's teased this project for a while, it seems. Here's what he said at E3 2010.
After listing these activities, he gave some solid hints about some of his upcoming plans. "I think that next year I can probably deliver something new," he said. I'm still active on the development scene, so please look forward to [my output]. It's been nearly 10 years since I made the Pikmin character, so I feel this is the year where I must at long last make a new character."
Since Pikmn 3 was something he was heavily involved in, moved from Wii to Wii U and was delayed, I figure that his new project took a back seat, but now he's likely working on it full swing. Pikmin was really the last franchise we got from Nintendo for the core Nintendo fan. That was ~12 years ago. I don't think this project is going to be a Wii Music or Wii Sports type of project nor do I think it'll be something like Steel Diver.
I really do think this will be a brand new character for Nintendo. I'm trying to think of what type of genre Nintendo could cover with a new IP. I'm hoping it's some kind of multiplayer focused title, but who knows.