Just some ideas...
1. Hangar Damage System
Like a damage of fuel tanks effects on percentage of fuel, damage of hangars
effects on available plane types. Followings are the case of the middle field: FH.
[case 1] Damage and ENY
1 FH destroyed: ENY 0-15 disabled
2 FHs destroyed: ENY 0-20 disabled
3 FHs destroyed: ENY 0-25 disabled
All FHs destroyed: All fighters disabled
[case 2] Assign fighters to the FH
FH1 destroyed: N1K2, P51D, ...., 202 disabled
FH2 destroyed: F4U-C, La7, ...., 109f disabled
FH3 destroyed: Tempest, 190D9, ...., Spit V disabled
FH4 destroyed: Ta152, Spit IX, ...., A6M disabled
(Plane set is just an example.)
2. Planes and Vehicles Factory
Place 20 to 30 factory buildings on this field. Also two cases just like a
hangar damage system is applicable. Some kind of planes should
not disabled even though all the buildings are destroyed.