I was able to get a decent fun kart map working, but it ran into a few issues:
A) To my knowledge, you can't control what direction people will spawn into their carts at. After the first round on the map, everyone will spawn into their karts facing whatever direction they were when the previous round ended. This might be fixable, but it was annoying.
B) I couldn't find a reliable method to remove the parachute from the jump spell. This might also be fixable? I was able to accomplish it with a point_clientcommand, but those don't work on all servers.
C) Double-shot bug as mentioned before. Testing a kart map basically means sabotaging an entire map test.
D) It gets old after a while. It doesn't lose its novelty as fast as medieval mode does (for me!), but I don't think it's a sustainable mode. Interaction between players is fairly limited, especially when you're limited to two teams -
the best I could come up with was a time-trial mode where the first team to X laps would win.