A tunnel that splits into two sections that give slightly different paths to a jump given whether you're driving on the road or ceiling would be nice.
Best way it'd work is just have whatever that area of road is (rocky/snowy/ice or tunnel) open up into two paths stacked on top of one another. The top one you can only access from the ceiling. Gives a bit of unique variation in route and emphasises taking advantage of the fact the cars can drive on that stuff.
As well as taking advantage of how open-ended routing is in the game right now.
Some jumps up onto upside-down sections for pickups would be interesting, especially if there was a corner involved. ((Main problem is that the cars currently have a habit of just bouncing off ceilings if they jump up into them as evidenced by the abandoned building at the start of Figure 8, that'd need looking into if ramp-to-ceiling routes are to be a reliable 'cliche' within race courses
Also, how about some completely vertical descents? ((Like, the cover of Stage II always gave me the impression of the cars racing on a vertical incline from a higher section of a track to a lower section. Those insane speeds
especially if you put something cool like a big jump over a big sprawling pit of water/lava/something else that looks flashy. (of course you'd need to be able to drive through that part if you screw it up and crash, heh...)