I think having a black market would be a great thing to have. Black markets would allow you to sell guns which you can get by robbing houses, buying/selling drugs, buying weapons at a discount, and buying C4 for city bank robberies (check the suggestion here).
Black markets should be somewhere where players don't usually go so we can boost the popularity for those places.
Great spots would be:
LS -> East Los Santos or Jefferson Motel
SF -> Garcia
LV -> somewhere in the desert
If a cop sees you buying some things at the black market, you will receive 1-2 stars for illegal trades.