Idk if you are taking suggestions, but if so, I think you should do the mound builders. It's an ancient collective society all across North America, with a rich yet mysterious history. I imagine that they would have Mounds (UI) that provide culture bonuses every time a unit is killed, and Big Chiefs (Great General replacement) that can increase the attributes of cities in which it garrisons. UA I'm stuck on, but maybe it could be something along the lines of Mound Builder cities always have Fog of War (to opponents) unless a unit is currently in the territory, and Razed MB cities become ancient ruins. So idk if this interests you at all, but either way, "hit me back" as they say.