Love the mod.
A lot of great suggestions already listed.
Some Ideas for callouts that I'd like to see are Medical & Fire emergencies. Either escorting to the scene and/or arriving on scene to provide support (I like the idea of VIP escorts also).
It would be cool for random crimes to pop up right in front of you (traffic violations, fights, muggings, etc).
Another thing that I would REALLY like to see is improved AI around your police vehicle. A lot of times if you have your car parked with the lights flashing, the AI drivers will get impatient and run into it, go around it on the sidewalk, etc. I'm not sure if it's possible, but maybe have it programmed like the AI works when you are actually in the vehicle with the lights flashing. AI drivers seem to move over to the side of the road, stop, and generally avoid the car then.
My biggest wish would be for Rockstar to bring you guys on board to make another version of GTA where you can also play as the roll of the cops and have a retail version of what you guys are doing here.