Nintendo Working On Brand-New 3D Super Mario Game For Wii U?

[Soundwave]

This was basically confirmed in the Iwata Asks for Super Mario 3D Land, that both the 3D (EAD Tokyo) and 2D (EAD) Mario teams would take things from the development of SM3DLand for their next projects, Koizumi is EAD Tokyo (3D Mario) and Tezuka is EAD (2D Mario):

http://iwataasks.nintendo.com/interviews/#/3ds/super-mario-3d-land/1/4

Iwata

It is satisfying both as a showcase of all that's good about 2D and 3D and as the grand sum of what you've done so far in 3D.

Koizumi

Yes. The fun contained in Super Mario 64 and Super Mario Sunshine is scattered throughout, so it's fun to look for those places. I think this game will be key in leading to the next Super Mario game—a new point of origin, or a crossroads.

Iwata

I see. Tezuka-san?

Tezuka

This game feels like a collaborative project with 2D Super Mario, so it introduces various devices and enemies from 2D Super Mario, as if to say, "This is how they look in 3D!" I feel like we are introducing something new to players of 2D Super Mario. I want to reflect what we gained from Super Mario 3D Land in the next 2D Super Mario game.

Iwata

Both 2D and 3D Super Mario games may receive an influence from and build upon what appears in this game.

Also just a FYI, SM3DLand was made by a subset team of EAD Tokyo, not the main group. The main group has not made a big game since Super Mario Galaxy 2 wrapped in 2010. And really SMG2 was basically ready by late 2009, they opted to move it into 2010 to give NSMB Wii space.

Super Mario 3D U for 2013 would be highly, highly likely. I suspect Donkey Kong Country Returns 2 will be ready to go for 2013 too.