The Forgotten Realms or Greyhawk,
I could do with with you @Bannacor, I am sure there would be interest. I am more familiar with the Forgotten Realms also.
My quick thoughts, would be an up and coming gang. In the streets of Luskan, BG, or Athkatla. The gang, the group, could have ambitions beyond the or wanna keep it small but do larger jobs. Maybe be a gang on independent contractors that work with the highest bidder. The Campaign would be focused around your criminal empire and reputation increasing, build connections etc.
Adventures could range from robberies, debt collections, bribing, smuggling, assassinations, piracy, highway hits, fencing, forgery, burglary, etc.
If the group is or would be interested a hideout, building a crew/guild could also be fun. Or keep it simple just a gang doing their thing using connections to hide from the guards etc.
let me know if that is what you were thinking. In my opinion a total evil campaign done right could be a lot of fun.