Miyamoto: Just so there is no misunderstanding, I should point out that this doesn't mean we'll never make another Super Mario Galaxy game.
Koizumi: That's right. When we first started making Super Mario 3D World, Miyamoto-san asked me if this was going to be more like Super Mario Galaxy or more like Super Mario 3D Land. When we made Super Mario 3D Land, we had our eyes on the form of this game, so we made it this way without any hesitation.
Miyamoto: The same team can't make both at the same time. And we can't bring in a second party and slap the name Super Mario Galaxy on it. I suppose we could idealistically make both in Tokyo, but we want to do something new too, so there was that dilemma.
So it sounds like they want to do Super Mario Galaxy 3 too some time. That is pretty interesting to me.
This all comes from the Iwata Asks for Super Mario 3D World.