UK:
Hawker Hurricane or Hawker Sea Hurricane (assuming we're going to see WW2 aircraft carrier operations in the future).
Hawker Typhoon.
Bristol Beaufighter.
Fairey Firefly (again, assuming we see WW2 carrier op's in future).
Germany:
Junkers Ju 87.
Henschel Hs 129.
Messerschmitt Bf 110.
Messerschmitt Me 410 Hornisse.
USA:
Curtiss P-40 Warhawk.
Bell P-39 Airacobra (could arguably be assigned to Soviets via lend-lease due to it being favoured more by Soviet pilots than American).
Lockheed P-38 Lightning.
North American B-25 Mitchell (I realise this is classified as a medium bomber, however, it's my understanding there were numerous variants some of which were more akin to ground attackers).
Russia/Soviets:
Mikoyan-Gurevich MiG-3.
Yakovlev Yak-9.
Lavochkin La-5.
Ilyushin Il-2.
Japan:
Mitsubishi A6M Zero.
Nakajima Ki-43.
I realise it's quite a large list and specific variants haven't been included. One thing I hope to see is liveries included with each aircraft for the major nations that used them which have been announced/introduced. One example would be a Fleet Air Arm livery for the F-4 Corsair when it is eventually introduced. The same would/could apply to aircraft such as the Hawker Hurricane with a Soviet Air Force livery.
I also realise that I've only included a couple of Japanese aircraft, admittedly the Pacific theatre isn't my strongest area of knowledge. I've also omitted the Italians for the sake of trying to avoid getting carried away, however, if we were to see an expansion of operations into the Mediterranean and North Africa then this would open up the opportunity for some additional interesting aircraft.