Sustainable Scenario

[Shakeshake]

The user made add-on "Cities of the Frontier" plays sort of like an endless scenario. It has city building elements as well, but essentially it comes down to endless enemies coming to pillage the "city" you've constructed (you can "win" by collecting enough gold--you may be able to continue playing after meeting this condition, I'm not sure, I haven't really focused on saving up--but either way it's easy enough to avoid getting enough gold)!
It's not really a case of evenly matched teams battling endlessly in perfect balance (which I imagine would be difficult to construct when both randomness and AIs are involved anyway), but it seems fairly close to what you're looking for if you just want a continuous struggle against perpetual enemies.