Hey, look! It's the dread of every novice Mario Kart player. The rail-less, multicolored corkscrew better known as Rainbow Road.
Todays screen shot reveals that (just as many predicted) Rainbow Road will be a new playable stage. Though it appears it may be a 3DS exclusive, as the stage looks very much like the Mario Kart 7 track. The stage also appears to be a "cruise stage" like Mute City or Delfino Plaza (meaning moving platforms and changing backgrounds).
Are we witnessing Rosalina's upwards smash attack? Her recovery move? It's very possible. Poor little Luma looks like he's about to get a nice face full of missile in just a moment. Watch out!