Hi! I thought of having multiple cities the size of Constantinople or ancient Romeback in the Empire for a setting I was doing, and I thought about how amazing multiple walls would be. though the first thing would be to add an "enormous city" option which could house ~a million people.
I thought of a city with concentric walls. A bit like Minas Tirith with an outer layer of very thick walls and inner layers with gradually increasing wealth. maybe with an especially large citadel.
Of course I have no idea how difficult this is to do, but I definitely think it would be an amazing addition to any city map, especially one as good as this one!