This would apply to those of us who create custom arenas, mostly...
Add two attributes to each plane/vehicle: Era and Side
Era - which WWII era the plane belongs in (was introduced), EW/MW/LW
Side - Axis or Allied
Example:
Someone wants to create a quick two-sided Early War custom arena. They could go to the fields screen and select [Bish, Early War, Axis] and all Bish fields would have the appropriate Axis equipment that would only be available during the Early War years. Then they could select [Knight, Early War, Allied] to assign the Allied side. They could then further tweak individual items.
