This is a non-linear adventure, good as a sideline for whenever the PC's happen to be home.
The PC's are based in a large city. The city is basically composed of three sectors. Two of which are virtually lawless and the other is extremely well controlled. The law portion is extending outward and slowly taking over the other two sections.
A faction war is taking place in the city. There are two opposing forces at war with each other (it could be a peasant/slave revolt, or a religious purge, or a supernatural invasion, or whatever.)
The war expands steadily, more and more groups getting dragged into it and being forced to choose sides. An interesting twist would be for 2 groups that 2 different PC's belong be on different sides. Great chance for roleplaying here!
The war could develop while the PCs are away, and upon return they get the opportunity to jump in.
Think of it, the politics! The adventure! The intrigue! The danger! The chance to be hunted by one of the most powerful groups in the city/county/country/kingdom!