Basically, a Script where the player can go to the police station and become a cop as well spawn a police car when speaking to the Ped in the police station. Also have the locker room to refill ammo as well going on duty gives you all the weapons necessary such as a pistol, baton, taser and much more. Of Course to go off duty just go back to the police station and go off duty.
Secondly, that ties into my another idea is when going on Duty there be a Police channel only where you going to be talking to officers only and be talking with them when going on duty. Should also be a way to turn off the channel or AKA radio and turn it back on in case you want to talk to a civilian in a pull over for an example. Could be also expanded with other factions like EMS.