Could perhaps do a parkour-themed map that I'll tentatively call "Spaghetti and Meatballs". The parkour track would be so convoluted and entangled with itself as to be a little difficult to see things clearly (lots and lots of distractions and blockages.) A vast pool of water (if you did a modded map, you could use a more appropriately-colored liquid) would serve as the sauce and prevent those who fell off from dying from the fall. This area would be densely populated with spawners, making it extremely difficult if not impossible to survive long (hence the "meatballs" part of the title).
Another map idea could be the inside of a giant-scale mob grinder, or maybe a normal-scale one that stretches on for forever.
I wonder if a map designed around horse breeding and racing could be pulled off? There wouldn't be much point in it, but who knows? Maybe you'd be able to start a new genre of horse-parkour mapping?
Edit: another idea just popped into my head, the coin-operated world. Every new area would be unlocked by a new farm, and it would cost produced items to be able to stay in that area.