How about a "Speed" theme, where once you get over 50 mph (just using the speed from the movie as an example here
) you can't go below that speed for the rest of the track, or else BOOM!
The speeds would have to vary depending on the genre. They would have to be high enough so that they're somewhat challenging to maintain, otherwise the maps wouldn't look much different from normal ones. And as a rule the minimum speed should be reached within ~10 seconds of the start.