This is a discussion I was having with a fellow enthusiast on an automotive forum, but I think it would be really cool if Ben could do a vehicle-mounted RPi system that could do various things like control relays to turn on various 12v light bars on a vehicle, or even something as simple as using an ELM327 or tapping into the CAN BUS system to intercept the signals from steering wheel controls and turn it into an entertainment system / head unit for the car audio.
I'd really like to see something akin to home automation but in a car, so maybe the Pi can trigger lights on the car or the door lock system if it gets a bluetooth connection from your phone, or maybe it could log certain things on the car and then automatically send the logs over samba to a windows server upon wifi connection, etc.
I feel like there's a lot of cool things you could do with a RPi or beaglebone and an arduino board in a modern day vehicle with the newer can bus systems.