What will the Mario Kart 9 gimmick be?

Double Dash had double drivers, double items and special items

DS had retro tracks (technically super circuit did too but it was just the super mario kart tracks) and mission mode

Wii had bikes and half pipes

7 had underwater sections, fully customizable karts and gliders

And of course 8 had antigravity

So what do you think MK9's new gimmick will be?