I thought of this while building my last fortress. Instead of building a stronghold, I decided to try to build a partly-above-ground city. It turned out pretty impressively. It was well defended, and its craftsmen made the city filthy rich. I wanted to base the city's wealth on something exotic like silk clothing, but my master cooks ended up producing the most valuable goods, so I guess the city ended up being a giant restaurant instead. But eventually the king (well, vampire queen) ended up taking residence there! But as much as I wanted the city to be some magnificent capital, nobody ever came there but a few merchants and the occasional goblin horde.
So, I thought, what if the game tracked different types of traffic travelling around the world? I know plans are already in the works to track armies moving across the world, but it would be fun to see a variety of people regularly passing through your area. You know, merchants moving between civilizations, villagers taking goods to cities, soldiers patrolling, adventurers wandering around looking for dangerous areas, and so on. And depending on where your fortress is, you could encounter characters moving between destinations or even entice them to make your fortress a stop on the way to their destination.
If you built your fortress along a major road, travelers would pass through. You could build inns, taverns, shops, etc for them, or you could turn them away at the gate, or drop them into a pit and take their valuables, or whatever else you felt like. If you built your fortress near a frontier, scouts and war parties might come by to resupply. If your fortress was in a really dangerous area, adventurers would use it as a home base and even bring loot back to your fort to sell. You could even build your fortress in a mountain pass or on top of an existing road and charge a toll or completely stop anybody who tries to pass through and see how long it takes for an army to intervene. And so on.