It'd be nice to be able to duck down in a vehicle, rather then always being shoot through the window, when your have armor on your door.
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.