It could be any number of things.
Given starting conditions (or even give everyone the same map) see who gets the fastest win of each type in one week.
Create a ridiculously hard scenario and see who can beat it the fastest.
Fastest levelling of a champion to level 15 (turn count)
Best custom faction/sovereign design, where other players compete 1v1 against submitted faction/sovereigns and see which one has the best winning percentage. (Includes unit designs)
Stuff like that.