I have thought about this one alot given some of the circumstances I have encountered, and I believe it this can be practical in use. The ability to duck for cover while inside a vehicle. (depending on the vehicle as well)
The reason for this is when in a vehicle you are pretty vulnrable since you can't move, especially to any gunfire, and since people are more likely to aim for the player in the vehicle. You the player are pretty much a sitting duck.
Natural reaction to gunfire when in a vehicle is to duck for cover (lie down towards to oposite seat), that is if you aren't dead already, but repeated gunfire at the driver seat will pretty much ensure you are dead in seconds without a chance.
My suggestion is to map the prone button (default = Z) to duck for cover. So an example is, if you are in the driver seat, and you duck, you would lie down flat towards the passenger seat, and likewise if you are in the passenger seat, you will lie down flat to the driver seat. You could use this method with all seats, but limited to certain vehicles though as it would make no sense in helicopters, or in the back of transport trucks etc.
This will feel natural and instinctive since it is pretty much natural to do so in reality, but also in arma when under fire, instinctively we would prone... so the Z key would be ideal.
Now also, I would like to suggest a second feature, I will post a seperate ticket for this here http://feedback.arma3.com/view.php?id=6706 , but the reason for adding it here is because it is related. MadDog can remove this if he feels it is not required.
I would like to request getting out the vehicle safely without revealing myself.
as an example, if I am under fire from the drivers side, I want to get out the vehicle from the passengers side.
This can be achieved in combitnation of above mentioned ducking within the vehicle feature. If I the driver sits up normally, I would get out normally by the drivers side door. However, If I am under fire from the drivers side and I duck, leaning towards the passenger seat and then exit the vehicle, I would get out the vehicle on the passengers side.
This way when under fire, and the vehicle is disabled, we can at least get out safely on the opposite side and use the car as effective cover.