Unlimited provinces, cities, factions etc. With a clear warning that adding too much will make the game unstable

But other than what you described, I'd love to see:

Option to give minor skin alterations to units recruited from specific province or provinces. Like Rhodian motifs for hoplites from Rhodes and Athenian for hoplites recruited from Attika.

Rivers can be customized for width (the Nile river looked like a stream, well all rivers did).
Cities can be placed on top of a river, with city area on both sides (Memphis for example) or just one.
Coastal cities that are besieged will have no turns to surrender if their harbour is not blocked and there is a nearby friendly city trading with them (representing food being shipped in).

Think there are many more features I'd like to see, but this is what I can think off right now,.