According to an interview with Zoomin.TV, Shigeru Miyamoto is currently at work on an unannounced, original game.

Miyamoto himself didn't say anything about the game, so we have nothing else to go on: no format, no genre and no hint at which of his hobbies it'll be based on.

Miyamoto is still working on Luigi's Mansion 2 for 3DS and Pikmin 3 on Wii U.