(Taken and slightly edited from my old "The Sewers" thread)
On Era PC, there was a sprawling sewer system that served no purpose aside from a way to travel without being tracked on the map.
Why not add our own sewers, and put some meat on it as well?
Basically, the sewer is where the most unruly of degenerates have taken refuge, and have formed a strange community of deformed, insane individuals who can trace their lineage back to outcasts of the surface world. Various vendors peddle scavenged hats and questionable items in little caves dug out in the sides of the foul smelling waterways. To most people, the strange, scary and occasionally unethical things that happen down there are little more than urban legend.
The sewer-dwellers have strict rules, and the players is assumed to follow them. These rules are simple- no firearms, because they are loud and often frighten the unusual inhabitants of the underground civilization. No outside money, as oddly enough the sewer dwellers are frightened of anything produced by the civilization above their heads. And finally, outsiders are considered unclean and are treated with respect only when they bring "money" to the black market economy.
So what exactly is used for money here? Blood, flesh and bone. The sewer dwellers are cannibals that trade only in body parts. Except bone. Bone is used for tools and weapons, like metal is back in the world of the usual and mundane, rather than eaten.
So what am I suggesting? A new town in the sewers, where only melee weapons are allowed and things are... Unusual and almost horrifying. And maybe some new melee weapons, to balance out with the powerful stuff people can already bring in. This can be a major project to work on after some of the other big things like countryside are out of the way.