It has been proposed many many times in the past. But now that new vehicles and planes are being added I'd like to repropose this idea.
Howitzers, Pak's and any other gun you can think of is fair game. Each gun is towed around by a Jeep or truck or something. You have to unlimber it to fire, and limber it to move. Each of the guns is very deadly, yet very vulnerable.
There are a bunch of damage zones for the gun.
1.) Targeting / Aiming
2.) The gun itself
3.) The truck
4.) Gun crew members.
Now, every gun has a certain amount of people to operate it. As you lose more people on the crew, the operation becomes slower and slower. Up until the last person dying.
For the howitzers, I don't believe that they should be able to click on the map to aim. This would be too easy. However, I do believe that they should free aim with varying degrees of speed in which the gun moves (Maybe tied to throttle control?). Also, the ability to aim at specific areas VIA typing in the coordinates.
Of course I'm just brainstorming here. I think this would all be cool.