What I loved most about Saints Row The Third was that emergency vehicles could be customized, even to the point where you can remove the lightbar and make it a slick top cruiser.
Why can we still not do this? It makes no sense at all. It's not like there's a vigilante system that could be hard to mess with for custom police vehicles.
It makes no sense why we can't take, for example, a regular Vapid and add lights and sirens to it.
I get it, GTA is a game about crooks, not playing police, but so what? Give us the option to customize police vehicles if we want to.