Okay, what we have so far:
1. The Kuriotates are going to be about less, more improved cities.
2. They will be unable to build more cities because? Maybe they start with 3 settlers and they can't make anymore? Or we block settler building after they have a certain amount of cities based on the world size.
3. What about taking over cities, they don't seem like the sort that would raze cities. Maybe whenever they take over a city that city is turned into immigrants which the Kuriotates can use to add to their cities population or for other effects (seems to be more fitting with the creation theme).
I tend to not want to just apply maintenance modifiers to force players into sticking with less cities. I worry abotu the AI's ability to deal with that, but more than that it will feel like we crippled them to casual players. I think specific restrictions are generally better than punishing options.
4. Increased traderoutes.
5. Upgradeable improvements.
6. The biggest limitation of their limited city model is that they wont be able to spread to get resources. The chance of being able to get reagents, mithril, mana, gunpowder, iron, etc is very low. So they will need something to help them get the good improvements other than spread.
Maybe they can use their immigrants to build settlements? Small cities that cant build anything but are enough to put out a little culture so they can claim land with them?
7. Centaurs- funtionaly mobile archers to help defend their kingdom. The defensive moves will be even more important if they have to cover a larger area with the non-unit producing "settlements" on it.
Other things on the table:
8. Crafting- making raw resources into manufactured goods.