There could be research facilities, which would introduce new planes and variants to the planeset after a while, maybe even V2's, A-bombs etc (Any favourite 1946 what-ifs you can think of), if given enough time. Every respawn of a map would leave all sides with just the basic planeset. Just like bases, these research facilities could be captured. For the captor, it would be of course ideal to get an intact research facility instead of a leveled one..
There could be factories, which would resupply the airfields with planes and equipment. If a resupply convoy or train gets shot, the factory must rebuild the train/convoy first and then the resources for the airfield.
Another thing that I would like to see is a modifier to aircraft engine performance, bullet speed, dispersion and damage resistance depending on how badly or well the side is doing, reflecting the availability and quality of materials. Having that particular resource site down could for example mean lower quality fuel and reduced engine performance.
