We all know bandits from the Adventure Guild. How about a hunting ground for a secret place where they congregate and plan?
There has to be some form of fence, a black market and a bit of organization for them somewhere, right?

Example, in the wilds is a certain tree that has a network of branches which to the subtly trained eye reveals a pulley system for opening up a hatchway that even a large one could manage to get in and crawl through. It would require a lot of speed and skill to yank the lever-branch which activates this quick hatch entry system, then if they make it without taking any damage they are inside the tunnelway which leads to a small maze network of crudely carved tunnels. At some point, the adventurer can stand up and begin to make decisions about whether to reveal themselves or stay hidden, go off into another of network of tunnels when they happen upon the nexus of the operation. There is a door with a magical seal on it (presumably set up by some powerful magic user who was either hired for the job or runs the headquarters still there) where the bosses probably meet and where certain treasures, weapons and armor, plans, etc. would be stored.

Smuggling tunnels with a huge band of brigands could be fast and fun! There would also be some unusual creatures under the ground, you might find r'oater cousins or animated rocks, etc. I once posted about large slimy grubs and huge gremlins, this would be a great place for greater gremlins which try to take your magical objects like wands and use them against you! Haahha.
Presumably on their own turf they would be better equipped and capable of having many many forms of traps and fake doors, things like that. It would be fun to have little clues that there's a racket going on with the politicians and businesses of local cities, something always just under the surface but never quite visible.

Dragons r Real