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I have to say that there would be one minor problem with retinues: the commanders are unlikely have children...

Also: I think the different castle upgrades (like a large eastern castle) are already giving some kind of diversity in the game

But that said: I would be all in favor of having a bit more ditinction between the cultures!
 
I like Vanilla having very diverse armies why not add this to this mod? Knights to the south, Crannogmen, Clawmen, Unsullied, Norvos Axemen, there are many possible units that could fit into this. Add more variety to the different cultures it makes the game more diverse.

You should go check out the sub-mod boards on the Citadel, the mod's web page. Can't link it, unfortunately, but there are several sub-mods that would work just dandy for what you're looking for (though you may have to change some of the retinue names to your liking in the localisations folder).