Hi TC!
IMHO, these would be essential:
Fighters:
Bell P-39 Airacobra, Curtiss 75 / P36 Hawk, Dewoitine D520 and Polikarpov I-16.
Special models:
Fairey Swordfish, Fiat CR42, Gloster Gladiator and Polikarpov I-153.
Transport models:
Junkers JU52
Light Bombers:
Curtiss SB2C Helldiver
Medium Bombers
Bristol Blenheim, Heinkel HE111, Mitsubishi G4M "Betty", B-25 Mitchell, Petlyakov PE-2, Savoia-Marchetti SM79 "Sparviero" and Tupolev TU-2.
If these models were introduced, AH planeset would be able to cover the whole 1939 to 1944 period, giving the Finnish, Soviet, Italian, French and Japanese airforces a better setup for historical setups. It would also give to British, German and American airforces some important models, closing many gaps, like the lack of the HE111 or the Hawk.
As you may have noticed, since I formulated this wish, we allready got the B-25. Only 16 to go...
Sparrow