I'd like to see a device that lets the car be driven for 5 mins, then locks all doors, sprays passengers in clear liquid or a dye that 'taints' them as having been in that car, then shuts down the engine and sets off alarm. And calls police, sending a pic of people in the car and their location.
They'd be far away enough by then not to hurt the owner and the owner could have an extra wireless key that disarms the system when they're using the car. But the car will still start without it, only the system will be armed and active, with a audible warning a minute before alarm goes off.
Could have a manual override by entering pin via the radio buttons, if the disarming key was left at home or forgotten.