The ability to convert a personal vehicle into a cop car (Lights / siren and

The ability to convert a personal vehicle into a cop car (Lights / siren and the robber visibility range)

It would be really cool to have your own customized personal cop vehicle. It would avoid people fighting over the higher end pursuit unit cars and would allow for more incentive to invest in property / vehicles.
I think cops going around in a perrenial would be a bit stupid to be fair. The reason there are only 3 pursuit vehicles is because they give the cops an advantage. You would end up with every single cop driving around in an Infernus and it would be unfair. Sorry but it's true.