Just saw this on Newegg (mind you, it's NOT the official product description from Nintendo
Mario Kart 8 Wii U is a new game for the Wii U. It features new elements such as customizing bikes and designing your own tracks. It is the sequel to the game Mario Kart 7.
Not sure what to make of this...
Satoru Iwata did say Mario Kart 8 would be (one of) the first game to really push the GamePad
[can't find exact source, so disregard this please], and a track editor would be one of the best users of the GamePad so far. The part about customising bikes kinda makes sense, since you could already customise karts in Mario Kart 7 (but there wasn't any bikes in this one).
Honestly, I remain skeptical, but who knows. They've been pushing Miiverse a lot (