I'm pretty sure another 3D Mario is in development as we speak. I'm not sure if that means "Galaxy 3" exactly, but I'll just refer to it as such as it would be spiritual sequel to the Galaxy games.
Note that Miyamoto has gone out of his way to say a "Galaxy 3" is something that could still very well happen and usually when he starts making statements like that it's kind of a wink, wink, nudge, nudge clue to mean "we're already working on it" (just like when he was saying Pikmin sure would benefit from HD, and Pikmin 3 was later announced as a Wii U game).
Also oddly Mario 3D World won't have any DLC like NSMBU did, which to me suggests they are already working on another Mario game as 3D World wrapped up, and are taking any ideas left over (which seem to happen to EAD Tokyo a lot) and simply putting them into the next Mario game rather than considering DLC.
And just the tone I get from the Iwata Asks on 3D World is like "we were going to make Galaxy 3, but wanted to do this detour of a multiplayer 3D Land for consoles first".