What about if after we "retake the city" in-game event via expansion release, as time passes new content gets added to the retaken city and appears as if it's "being rebuilt". This would mean players would have to work together to fight off the Besieged enemies and the better we all do during the fights = how much damage gets done to the city. If we lose the fight we could even be pushed back out of the city and forced to re-group.

Lots of possibilities with this. I loved Besieged, I'd really enjoy seeing it redone on better technology. (I still remember the sad days of dial-up..)