If you can provide us with some historically accurate suggestions for any of the civs please share them with us. Saying there is a lack of models and not providing any constructive information on what could be done instead is counter-productive, especially on an open-source game that's still in Alpha.
Using flags like that is not a good idea, they would only be confused with garrisoning. I wouldn't mind something subtle like using banner or statue props: more or fancier banners could indicate a later phase. I'm not sure it would look good with every civ's style though.