I would still like a bit of a revamp to the aircraft.
Early pistern engine fighetrs (WWI, eg sopwith cammel) -> Intemdiate pistern engine fighters (early WW2, eg Spitfire) -> Advanced pistern engine fighters (Late WW2 P51) -> First gen jet fighters (Late WW2 ME262)-2nd Gen fighters ( eg F-105) -3rd Gen fighters( eg F4 Phantom)- 4th Gen fighters (eg F16) -5th Gen Fighters (F22).
Carrier fighters Early (WW1, eg Sopwith Pup)- Late (WW2, eg F6F Hellcat) - early Jet(eg F9F)-> Middle jet( eg F14 -Late jet( eg F35)
Early Tactical Bomber (eg Airco DH.4)- Middle Tactical bomber (Ju 88) -Late tactical bomber (F-111) -> Advanced tactical Bomber (A-10)
Early Stratgic Bomber (eg Gotha G.IV) - Middle Stragic bomber (eg B-29)-Late stragetic bomber(B-52) - Advanced (Eg B-1)
Special
Stealth Bomber (B2): should be a expensive and resourse intensive class not easly obtained.
Stealth Fighter (F-117) Like above.
Dive Bomber (eg Stuka) WW2 era tactical bomber good for killing tanks and infantry. Upgrades later into either fighter or tactical bombers.
VTOL (Eg Harrier, F35B) Can fit double the amount into aircraft carriers, not as good at air to air fighting.
Also as you progress through aircraft tec the cost should increase dramatically so you have to choose if you want a huge airfrce of older aircraft or a small airforce of more advanced aircraft.