I'm a driver/pilot enthusiast, so I was quite frustrated and enraged when I couldn't fly the darn bi-plane through the kitchen window! I can do it now, but it is very difficult to pull off because you have no feedback on your control mechanism, the mouse. The problem is I can't tell which way I'm turning untill I'm already doing it. With a joystick, I know if I point it left, I'm going to go left. With the mouse, if I move it left, I have little clue as to whether I'm really going left, or just less right.
This problem has a simple, universal cure: show the mouse when driving a vehicle. Any vehicle. A simple "third party" mod, indepentant of the vehicle, can do this simple thing, no? This is the most basic way to fix the problem; I see the mouse, I know which way I am turning/going to turn, and by how much.
A better solution would be to put some sort of "aiming reticule" in place or the mouse's position, maybe with a line leading from the center with a color code (maybe green-yellow-red, blending would be nice) to see the intensity (distance from center) of the turn.
With this ability, you can fly a plane carefully and precisely, mastering the control and pulling off moves you didn't think possible. I'd write this code myself, but I just don't have time to learn the language.
So, I'm thinking something like the background music mod; you press a hotkey, and presto! The steering gui is online, whether you're in a vehicle or not.