I think this change will be much more intimidating for drug dealers in general, as well as offering/cutting different approaches to raid these "drug" places.
Now, about the drug delivery. I will be using two references from Breaking Bad and Better Call Saul TV shows, because they are most logical for me. Possibilities for what could be in the actual game are pointless, but that's beside the point.
- After you pay the dealer, he will signal his business colleagues to deliver the drug by the car. They will drive up near the player and you have the ability to take drugs you ordered. After you do so, they drive off and the dealer will tell you to leave. If you keep staying near, he will get annoyed>hostile, each phase taking 15 seconds.
This variant would work best if the dealer is outside, otherwise it wouldn't make much sense for them to just not keep the drugs somewhere inside or near the building. - The delivery men/man will now drop off the drug in some quiet, secluded place near the building. For example, he will slide everything you ordered down the drainage pipe. While he does so, the dealer will tell you where to pick your product up.
About the possibilities this can bring.
You can have cops called somewhere around the area, to them see how a suspicious man just dropped off illegal substances at a suspicious spot, so they will obviously try to arrest/kill him.
You can set up the same cops or other people to start a shootout once the delivery men arrive, to have them more clustered, you can use items that benefit from enemies being close, like bombs, and etc.
The way it's in the first Streets Of Rogue should be an option, where the dealer himself has product he sells, but the level generation should pick between these options to give players more unique experience.