Something difficult, less scripted, requiring adaptation and personal responsibility but also giving the ability to cover for others if need be, and not merely set in a flat, obviously circular or rectangular arena. I'd like to be able to use the environment to some advantage, rather than just right-clicking on a cannon or such tool at some point. I don't really care against who. Would not be willing to rush in a boss idea that would have better fit a later expansion.

Doomtrain could be pretty great if done well. Preferably in a fitting expansion.