One feature I would like to see from firing from vehicles is the ability to shoot out of more than just the back of vehicles. For example, the passenger of civilian vehicles should be able to fire out the window, and the copilot of an open-cockpit helicopter such as the XH-9 series should be able to fire out of the cockpit (video in Add'l Info).
The passenger of a car should be able to fire out of their window. But this is a very limited area they can fire from, so the person could press, say, LCtrl+W and their character would rest their armpit on the window, lean their head out, and hold their gun outside the car. This would allow them to fire at a much wider area than before. However, the penalty for moving into this less stable position is that the weapon sway, effect of recoil, and misalignment of sights from movement both the weapon and vehicle would be much greater, significantly hindering accuracy.
In the same vehicle, the player would move their stance adjust once again (this time, say, LCtrl+S), and their character would move the gun to their other side, swivel their body and lean out the window, but this time, because their seating position is an awkward one, range/speed of motion and accuracy would be even more affected.
This applies to other certain positions also. For example, while sitting in the back of a pickup truck, the shooting position is fairly comfortable/natural. But say the player wants to get a slightly higher vantage point for whatever reason. They could stance adjust upwards, granting them a higher position with more freedom of movement at the cost of exposure and shooting stability. Conversely, under a hail of fire they might stance adjust downwards to conceal themselves within the bed of the truck (also good for hiding in general), at the cost of field of view.