I have played this mod for a few rounds now. And I totaly love it. It gives more diversity to the game. But: ADD MORE SETTLMENT PLS!! :D :D :D

Some tips:
As you said military settlements for each faction.
Good stuff might me:
+1 reqruitment
+ ??% researchrate rate military tree
+ Walls
+ Somethins special that is to each group (slavic - barbarians - romans ect)
+ Better garrison
+(last upgrade/upgrades) experience for troops
+ extra income from tax in region
- HUGE openelty on food an squalor (since military settlements will be very good)
extra
+/- very very smal happines and growth in any at all (maybe negative happines?) i mean happines comes from military precence.

Cultural settlements
+ income ??% for every cultural building in province
+ ??% research civic tree
+ income ??? from culture
+ Culture give happines right? so maybe happines
- Squalor and food ofc.

Trading settlements
+ income ???? from commerce
+ income ??% from traderecources
+ income ??% from commerce in province
+ Trade capacaty (how many you can trade with? if that is a thing)
- squalor and food


some other suggestions would be:

Industrial cites, same as Cultural and trading, but with a higher bonus "%" for industrial.
The main thing is beacuse I myself never use industrial at all, almost. Like worshops and stuff. Most income is always agiculture.
But building this with maybe a gold mine should have huge effect.

Maybe relegiuos settlements? penelty there may be uppkeep?
But HUGE happines bonust and bonust towards your relegion. This one though would need one settlement tree for each religion.