So I was driving (in real life) earlier today and saw an undercover police car drive past me. They were driving an orange van with no markings, but had red and blue lights in the grille and on the inside windshield. I thought it looked so cool -- and then the idea dawned on me. It would be awesome to see this in GTA V!
It would be like this (feel free to post your ideas, too). During normal gameplay (no stars) every so often (maybe 5-10% chance) a ped will spawn in, either walking or driving a [seemingly] regular vehicle. This ped will take the role of a police officer and will use the voice of one, taking into account the appropriate gender. If you commit a crime with an undercover cop nearby, they will react exactly how a regular cop would (i.e. give stars, aim/fire gun at you, etc.). The undercover cops appearing as regular police blips on the minimap could be optional.
The cars they would be driving would be, as stated in the previous paragraph, the cars regular peds use. However, and this can be optional, they could have a speed boost, making them faster than normal vehicles (maybe fastest transmission, turbo, strongest engine, or other upgrades).
I hope I explained that thorough enough. I would be very grateful if a modder out there could pick up my idea and turn it into a reality