FIX THE ROSTER
I might take heat for this, but I think the roster should try and exclude "clone characters" (characters who are literally the same people more or less), and even the random mooks and instead focus on only the individual, canon characters. Here's a solid roster using this idea:
Normal:
Mario, Luigi, Yoshi, Birdo, Bowser Jr., Boom-Boom, Pom-Pom, Cranky Kong
Light:
Peach, Daisy, Toad, Toadette, Diddy Kong, Dixie Kong, Nabbit, Kamek
Heavy:
Wario, Waluigi, Bowser, Donkey Kong, Rosalina, Petey Pirahna, King Boo, K. Rool
I think this nicely encapsulates the main Mario crew---and brings some extended members of Donkey Kong in.
Now, I think it'd be cool to incorporate the "Mook" characters into some DD-lite system, with the mook characters perhaps riding like in a side-car, or hanging on the back and them statistically influencing your Kart. This is where Shy Guy, Koopa, Dry Bones, Blooper, Boo, etc.
Mainly, I also want more single-player content---a hub world perhaps with various challenges? A human character (not a Mii, like the ones in the Tennis and Golf RPGs) to customize and serve as your player character during the single-player campaign, unique challenges, and of course Boss Fights like in DS. With the Koopalings, Mario 8 could've created a cool adventure mode, but they didn't, and my interest in MK8 quickly waned once I played through the GP a few times and did the DLC.
I really can't think of any actual race gimmicks---I'm fine with what they've got. I'd like them to explore more dynamic courses, and single-lap courses. I'm also a bit tired of Retro Courses, and hope the next Mario Kart is a "reboot" with 8 Grand Prix made of completely unique courses.