From an interview in EDGE :
Unfortunately, they don't say whether it's for the 3DS or the Wii U. Yoshiaki Koizumi confirms that if the next Mario game is on the Wii U, it'll use the GamePad more than 3D World did.
Source
“That’s still a secret! I can tell you, though, that we’ve already started approaching our next challenge.”
“There’s still a lot more room for discovery and invention, and we’ll continue to propose new and exciting game mechanics going into the future.”
“If follow-up to Super Mario 3D World is to be a Wii U game, co-director and EAD Tokyo team leader Kenta Motokura says his team “might need to make even more use of the GamePad.”
Unfortunately, they don't say whether it's for the 3DS or the Wii U. Yoshiaki Koizumi confirms that if the next Mario game is on the Wii U, it'll use the GamePad more than 3D World did.
Source