I sort of assume that the vast areas between fortresses and towns are not actually just trackless wastes. It seems like there would be people living there of the same types as the civilizations that control the area. When the army arc gets finished, one thing that I would like to see would be a forewarning of impending attack, however this warning is inherently a problem. That is, these people who live out in the wilderness would find themselves on the warpath of invading armies. Rather than sit there and be slaughtered, they would flee to places of perceived safety--fortresses.
As armies move around the map (it's listed that they will try to recruit as they go, dragging people along to attack places), some people should be able to avoid their press gangs and get out of their way before the army catches them. These people should flood to nearby fortresses and large settlements, flooding their halls with refugees. It would only be a temporary population! You might have a fortress of a hundred dwarfs running smoothly and self-sufficient. However, as a goblin army, led by a merciless demon approaches, you might get an additional 250 dwarfs showing up looking for safety and food. This ties into making sieges harder to weather, since vastly increased populations would strain resources much more and provide many more unsupported dwarfs to tantrum. At the same time, there are that many more people to fight off the encroaching hordes.
I know the FPS hit that would accompany something like this would be horrific. That's why it should happen shortly before a very large siege and should only be temporary, until it's safe for these refugees to go back home. Alternatively, I suppose the player could select who should leave and keep some of the more useful ones.
Ideas and criticisms?