I don't know if this has already been said, or if it's even a good idea, but here goes.
So, instead of simply razing every settlement to the ground and having AI resettle areas, I think it would be a good idea to have it so that you collect a certain amount of resources, like food for Skaven, in order to establish a permanent outpost/stronghold. This can be wherever, in whichever settlement you desire. The better the settlement however, the more resources you need to establish an outpost.
An outpost or wherever you have settled should give you a foothold in certain areas of the map; giving a boost to corruption and influence in the area, which maybe even could mean surrounding regions cannot be resettled unless your outpost is destroyed someway.
This enables the horde faction to semi-conquer areas, especially for strategic purposes. With resource management however, you wouldn't be able to simply conquer everywhere, so you'd have to be specific with establishing outposts for maximum effect.
I think if you're going to keep the horde mechanic relatively as it currently is, this would be something I think would be cool to see, especially with the Beastmen.