I was thinking about idea of robotics or another custom creations in the game. Like you go to shop, buy needed modules and create your own robot or machine that's going to do something(f. ex. money printer or a gun with more rare and illegal parts). After building machine you can program it in some sort of a graphical interface like you have f. ex. in UE4 so you make it work. And after you can sell it on legal market or black market for example.