Kingdom Hearts seems to be the franchise of the moment right now. Re:coded was just released in the West, Birth by Sleep Final Mix was just released in Japan and with Kingdom Hearts 3D: Dream Drop Distance on the horizon, you'd think that would be enough for the tim being.
Apparently not though, as it turns out Nomura's "mystery game" is in fact another Kingdom Hearts title, Kingdom Hearts: Birth by Sleep -Volume Two- a sequel to Birth by Sleep.
The game was revealed in the secret ending to Final Mix, but other than that, no details have been disclosed so far.
Source: Kingdom Hearts Union