First they should make the cops more smarter. In gta iv the cops were retarded. They should add weapons like tasers, tear gas, mase, and batons. They should also make the riots and riot police once in a while. You should be able to choose levels of force the cops use in the settings. They should not arrest you for hitting a civilian if they hit you first. When on foot and being arrested if you break it they should not shoot at you. Cops should also get calls and come patrol areas with more realistic and natural movements.
I would like to see about 15 to 20 cars. I also hope that there are different types of the same car (Different Rims, body style, etc.) Police forces should have different cars like airport security and highway patrol.
Police Crown Vic
Ford 1970 Shelby GT
Police Version of the Dodge Charger
Undercover police cars (FIB BUFFALO From GTA IV)
1960 Camaro SS
2012 Camaro
Ford SVT Raptor
Chevrolet Tahoe And Suburban
Lamborghini Gallardo and Aventador
Maserati Gran Tursimo
Porsche 911 Turbo
Lexus LFA and ISF
Bmw 2012 M5
Nissan Skyline R34 and Nissan GTR
Honda accord Sport addition
Hummer Police Edition
Mercedes-Benz GL, C class coupe, and C250 Sport Sedan