Oh, and here are my ideas for some retro courses.
Bone Dry Dunes
One of the features I plan to incorporate into MKX is stage shifting. As you race across the tracks, something is happening during the race. This makes it so that you aren't just racing in a course and racing in an actual Mario location. With Bone Dry dunes, what if the dry bone skeletons reanimated on the stage. You may have to make your way around them. Additionally, parts of the road could break as the dead skeletons are reanimated, resulting in more ramp timing on the later laps. You could also race across the ships coursing across the quicksand once the road starts to crack. And then, that big Dry Bowser Skeleton could reanimate, forming a massive titan that you get to race on. The idea of upgrading that stage to fit X-Treme's philosophy would be sick.
Super Mario Kart Amalgamation
The tracks in the original Mario Kart kinda reminded me of toy playsets, so I wondered... Why not make a toylike amalgamation of the Super Mario Kart tracks? On one point you could be racing Donut Plains... On another you could race across Choco Island... And on another, you could race across the original rainbow road. I think that'd be neat!