I think it would be a great idea to have mechanics as the focus. Say, try to make the best experience/levelling system or best RTS controls.
I say this because people here could really benefit from seeing how everyone goes about things. Since the competition would be for people only using C2, everyone could understand each other's work and really learn some stuff. It could also help form some standards for C2 use.
I think this way is just much better, as there are many game comps already and a few days to a week isn't enough to make a truly impressive game. But is enough to make a sleek, efficient mechanic with a basic example.