Hard to say really. As far as what Nintendo themselves NEEDS, a 2D platformer is the better option,
They need a 3D Mario launch title, like Super Mario 64, Sunshine, Galaxy. BTW it's not really possible to announce 2 sidescrollers at once (one for 3DS one for Wii u)
They need a 3D Platformer, that involves some exploration.
We need an explorable environment, not a "3D sidescroller".
Something like SM64, SMS, Banjo-Kazooie, Banjo-Tooie, you know, with the ability to exploratorily explore the exploratory exploration.