Every turn there is a chance that a rebelling free city is destroyed and replaced by a new city state. The newly created city state will get the nearby units of the rebelling free city and it will inherit the population of the old city.
Only cities that were no original capital cities and do not have any world wonder can be replaced by a city state.
Note that this mod increases the amount of city state players by 50% and then saves 50% of them in a pool, so they can spawn later in the game. In order to do that this mod changes their starting locations. In most cases the start positions of city states will be ok, if you do not use a huge amount of city states in your game.
On standard speed the chance of converting a free city to a city state is 7% each turn (scaling for other game speeds). If the free city is damaged this chance will be lowered.
See further informations on https://forums.civfanatics.com/threads/free-city-states.633375/
This should work with any other mods that do not modify the starting locations.
The total amount of city states will even further increase if you use this with my More Opponents mod.
Also I have tested this mod with CIVITAS expanded city states mod and it seems to work properly.
Special thanks to FearSunn who teached me some important things about Lua. Also thanks to some users on civfanatics like Gedemon who helped me.
Free City States 1.2
Gameplay, Rise and Fall