I vaguely remember something about Disney not wanting their characters being directly playable. Then again, Mickey was playable in Kingdom Hearts II. I think it's most likely that we'll play as Aqua (SPOILER)as she travels through the Realm of Darkness, where she will likely encounter King Mickey. In the Re:coded secret scene, in his conversation with Mickey, Yen Sid remarks that they only need to find Terra. This would imply that they have already accounted for both Aqua and Ventus. Not a big leap to assume that Mickey met Aqua in the Realm of Darkness.

Still, playable Mickey would be all sorts of win. Nomura did say he wanted to explore Mickey's journey to obtain the keyblade from the Realm of Darkness. He could do that from other points of view, obviously, but that would be the most direct way to go about it.