Thought a bit more about this. A system similar to the spheres of influence attached to the Shadow Powers in Svavelvinter would be very nice. It directly drives narrative in a fantastic way. It also implies that items that appear singular (fortresses) are in fact scenes of rivalry, which is maybe the Svavelvinter system's biggest contribution to narrative development.
PCs aren't that likely to gain a fortress on the day of the release of the main product. It might be better to push rules and extended stats for existing campaign "stuff" into a later supplement.