Character line up - use more actual characters from spin offs rather than generic enemies.
Vehicles - I hope the selection is more like Mario Kart Wii, with the very different handling of different vehicles, and custom colors for each character. Of course, they should take a look at Wii to fix its balance problems and make the variation actually worth it gameplay-wise, rather than a handicap for 3/4s of the vehicles.
I didn't care much for 7's underwater and gliding sections, but didn't hate them either. I just didn't think they added much. Either way, considering how the next title will be a Wii U game, maybe they could add some sections of races where people race "on" water to showcase a new water engine or something. Although I guess Mario Kart has never been a graphical showcase...
Reverse cups - This is something that should be easy to implement, but the series has never done it. Not all tracks would be compatible, unless they reworked them, but, still, there are always quite a few tracks that could work like this without any changes at all.
A campaign/story mode could be nice, but with the way this game is coming so soon there's no way it's happening. Really, even if it weren't coming so soon it likely wouldn't happen...
When they base levels on shopping malls and timber yards that haven't even been a staple of the Mario series you know something is up. There is still plenty to take from the Mario universe but they just for some reason do not wish to use alot of it. Hell, make a track based on the graphical style of Yoshi's Island, both new and old, just something original.
Although tracks directly based on game locations are always nice, some of my favorite Mario Kart tracks, design-wise, haven't been based on anything, like Wario Colosseum and Neo Bowser City...