I'm sure you heard of sirens being requested a million times, but why not add sirens that could be controlled by a UI app or by the keyboard? In the UI app, there could be an image of a siren box similar to the one below, with a clickable ON/OFF toggle switch, and clickable siren selection knob, which allows you to select between manual and airhorn siren tones, and the standard WAIL, YELP, PRIORITY (and optional HI-LO) tones. As for keyboard controls, the user could bind any key they want to that specific siren tone. As for creating siren mods, it could work similar to how creating configurations, except with the difference of using either .wav, .ogg, .avi, or .mp3 files for the specific siren tones. Please leave feedback, comments, or questions below.
On the UI side of things the game should not need any new code because it should already be doable the same way the radial menu works. Just a new app. I've got an idea: make a new "engine" (no flexbody, attached to nothing, multiple engines should be possible in 0.8) which when started with the app plays a siren sound instead of the standard engine sound. So a siren should be possible to create. And vehicle specific bindings could be used for keyboard control.
