I think the one I was interested in when I was younger, when MMO's were the rage, was to make a sort of MMO / dynamic war (probably heavily borrowed from ww2 online). So something like a combined arms warfare sim, on a persistent map.

The angle I had to it though, was the world was at peace for a thousand years. So no nation needed any armies or military, but are now suddenly thrust into a conflict. So the nations need to mature rapidly from basically a policy force, to a fully fledged military, integrating new warfare advances as they occur. With new advances and countermeasures coming out from each side as the game progresses throwing the state of affairs off balance. And then get's reset every couple of months or something.