Oh, I have some suggestions...
Custom Drop Point Destination/ Custom Drop Point Sector
Basically, if you die in a Custom Drop Point sector, You respawn in the tagged sector's waypoint (Which I assume will just be a teleport waypoint). Basically, a teleporter that acts when you die from a hazard in a sector and you respawn. Thi way, instead of going back to the checkpoint, you can go past where you failed and only lose time from having to respawn. This is mainly for Mario Kart mode, as all Mario Karts do this in their courses (For example, most given Bowser Castles in Super Mario Kart).
Oil Slick/Ice Slide
The current Ice instantly accelerates you to top speed, and are also terrible substitutes for oil slicks. Not good. With the Ice Slide, your controls would range from really tight steering to really loose steering with little middle ground, while Oil Slicks make you spin out if you turn or drift while on that sector.
Also, I think the hitboxes on the Item Boxes and the Koopa Shells could be a bit bigger. It's hard to hit with a green shell given how it barely slows your opponent down, and I find myself constantly missing item boxes when I'm practically in them.