The first thing you should do is probably to come up with an objective. In your case, something along the lines of "earn the most money" would probably be fitting.
By creating the end of the game and working from there, you have a specific purpose for everything you put into the game. When you have an objective for the players, you can create mechanics for them to reach that objective. If you go with "earn most money", you have already come up with this method: bringing "drug-cards" into play.
When this method is established, you can come up with various ways to manipulate it, like buildings and bribes. Then, you can come up with mechanics to manipulate those mechanics.
Each time you add a new mechanic like this, think about the following things:
-When and under what circumstances can the players do this?
-What components do I need for this?
-Is it worth it?
When you have some sort of outline for the entire game, test it and make changes until you are satisfied.