1) What about modifying difficulty settings ?
Like increasing city maintenance by 100 % or distance cost by 50 % for having smaller empires and more room for barbarians...
At the moment we can have an empire with 8-10 cities without being in bankruptcy without Courthouses or Markets...
You could add a new effect for a leader trait : - 50 % of city number maintenance costs by example...
2) Why not increasing cost of growth, like citizen consuming three "Food" instead of two ?
Cities would be smaller and more realistic in early times, because for long time only rome had one million of citizens... At cost of massive importation from over the Empire... Increasing importance of resources...
3) I suggest having food and production caravans, and of course commerce caravan...
A Food Caravan could carrying 20% of stocked food of its birthplace...
Ditto for a production caravan...
Commerce caravan could functions like in Civ Rev, at different rates depending from civics in place... Could be based on a type of civics...
4) is it possible to have a system based only with python for emulating revolutions, and revolting cities becoming Barbarian ?
At the end of Roman Empire, cities declared themselves "Barbarian" to keep from paying taxes to Rome, since Barbarians give no taxes to Roman Emperor...
5) That's all ! Good ideas ?