I feel like after how much I personally enjoyed(and still enjoy) my time with Mario Kart 8 that they should NOT go with some kind of major gameplay changing gimmick.
I think they have solidly established what makes the series so well regarded gameplay wise with that game and instead they should focus on beefing up the game's additional features.(Battle mode, more substantial singleplayer modes, etc.)
The roster is probably going to disappoint me in some way but eh, it can't be as bad as the 3DS game's roster... right?(fingers crossed anyway)