Everything Dangerous vs. Everything Else Dangerous.
Imagine, if you will. An RTS where you choose your faction. No, it's not a USA-USSR choice. Do you choose the modern US Army, Cold War-Era US Army, World War II army, or any of various choices? The numbers and scenarios would be designed so any match-up, like 17th Century Heavy Cavalry Vs. World War One France could be fair. A huge number of possible combinations, with some one-off "Giant" units. Could Godzilla defeat a mob of World War 2 Finns with Molotov Cocktails? What about King Kong vs. Pitchfork-wielding Transylvanians?
There'd be a faction system as well, so modern France and Britain will side with each other most of the time, unless it's a matter of choosing sides in a conflict between 14th century France and England.