So, I came up with a cool, easy mod for a fortress that I think would work.
1: Up the population of Rhesus Macaques.
2: Give RMs a gas syndrome that causes RMs only to breed profusely.
3: Give RMs a contact syndrome that causes slow necrosis in anything.
Hopefully, this would cause the surface of the world to become a blood-covered wasteland; your fortress would have to either abandon all trade, or, for more fun, build a decontamination airlock out of grates and waterfalls. Traders would slowly rot as they came in, and dwarves going to the depot would be briefly exposed, causing some damage, which you'd constantly have to fight.