CA sort of kind of tried this in Empire, and it turned out to be a critical failure. They were overly aggressive balls of fluff who hurled themselves onto the spiky wall of doom, aka the player faction.
But since ETW overall sucked that was just kind of expected after awhile.
But in Civ, this would make a lot more sense. The city states could function like the natives in Colonization, but in a much more advanced way. Hopefully.