Maybe a challenge with ride rails on normal maps (as you mentioned,) dash pads, or even both! I could see them being especially fun on Humpback and Robo ROM-en, and Bluefin.
An alternate take on special weapon challenges where you have the special active permanently like in some octo expansion stations would be really cool for certain specials like zipcaster and inkjet. I'd love if they did it for crab tank too since it would be really funny seeing everyone roll to the middle of the map.
For a more unrealistic idea, maybe a color chip challenge where each player gets a few random color chip powers like main firing speed, splash radius, moving ink speed, and more. Of course, they would have to be chips you could actually use... or not! Imagine extra dodge rolls on... a charger? Some would probably be off-limits since chips like poison ink, drone chips (unless we get to use drones) and lucky chips wouldn't really work, homing shots might feel too unbalanced, etc. This idea would never happen but it's fun to think about.