I'm actually rather surprised this hasn't been made, but then again it may well be that the talent pool simply isn't there for such an undertaking.
I know there are some mods that concern themselves with defense, but I'm thinking more on a dynamic ai-controlled/simulated fashion, not something the player sets up as a scenario within the game. I also know that there's that tower mod where you can attack a fortified tower and boss creature. That also isn't really what I'm talking about either.
I suppose maybe Millenaire might have something similar to that, but I don't ever recall the AI actively going around simulated or otherwise, occupying and conquering areas from it's generated point of origin.
It's really too bad nothing like this has been made, because it's just what the game needs to justify exploration outside of mere sightseeing or looking for game-quest related sites.