I've noticed a few niggles here and there (aside from battle arenas which is a huge omission) so I probably forgot a few:
Display race times
Option to show map on TV
Option to show only gamepad-player's screen on the gamepad instead of mirroring the split-screen TV image
Change position no. to "?" during maze section of Yoshi's Valley
Option to show more detailed kart stats
More battle options; stock instead of points especially
Battle arenas, preferably as free DLC















