I really like how cities look more advanced as they grow with new buildings replacing old ones so that a city immediately looks more developed than a village. However there is one thing missing, the buildings in a city still look like they are built on dirt. I would expect by the time you reach town or city level that you have paved streets or something like that. This should be a relatively simple graphics addition to make the terrain underneath a city upgrade visually with the level of the city.

Also (though I guess this is harder to implement) it would be nice to see some kind of houses spawn as the city grows.