OW takes place in a time of already established empires that you play against. however, perhaps, in every game, there's 1 or 2 "NPC" empires that aren't playing to win. Rather, like barbarians, they have a set schedule of them rising, peaking, but then, collapsing. so, not another "civ" per se in that they won't ever have a chance to win, but something to manage and balance w your empire's needs.
They'd start the game as the Great Powers of the Age. This is stage 1. They have multiple cities that are well-developed, maybe some wonders already (so, add more wonders
), and have a nice army. But, they keep to themselves, unless *overly* provoked.
After a set amount of turns (that is displayed like tribes), they advance to Stage 2: Expanding. Now, they are out to conquer and expand so beware. Think: rampaging barbs that can control cities a'la civ 4. Most civs will lose a city or two to them, ai and player alike. just hold on tight and weather teh storm.
again, After a set amount of turns: Stage 3: Decadence: cultural and scientific diffusion perhaps? infrastructure? something to mimic the advancement of the pax romana but also, increasingly high discontent in these cities forcing them to deal with all that entails, including the occasional city revolt.
Stage 4: collapse: civil war as their factions are in rebellion; their cities become ruins and/or controlled by Remnants of the empire that can be befriended, absorbed, conquered, etc. can be ransacked for scientific knowledge or culture, etc.
the "endpoint" for these empires would be in the midgame, although the remnants might linger on in a similar state to late-game tribes currently.