Great idea, D-D. One qualification: the aircraft type should have been flown operationally during WW2. Bombers and recce types shouldn't be neglected but I'll stick to fighters for now; also, to only the major combatant nations.
The Gloster Gladiator (or even Gauntlet, if you count the handful used in the early days of the North African campaign) to Gloster Meteor pairing immediately springs to the mind of this ol' Brit; splendid manufacturer, was Gloster, and the Gladdie would complement the Fiat CR.42 nicely in a North African scenario. The Italians already have the ultimate fighter in their planeset, so that's sorted, but how about the Fiat CR.32, which was still operational in 1940 (and would be a 'must-have' for any Spanish Civil War scenario, see also the Soviet I-15, below). The Germans can have a Jumo-engined Bf109D to add to the Messerschmitt variants they've already got, and just maybe the Heinkel He 162 Salamander. Japanese? Mmmmmm, got to be the Nakajima Ki-27, operational from 1937 and later code-named 'Nate' by the Allies (another of the Gladiator's opponents - in fact, the first Gladiator combats occured over China) but what might be the late-war choice? I'd go for the Mitsubishi J2M Raiden 'Jack' - or, maybe better, the ultimate development of the Nakajima Ki-43 'Oscar, the late-war IIIB, which could then be re-coded to create the I and II variants so glaringly missing in the present AH line-up.
The Curtiss P-36A was the first U.S. fighter flown into action by a U.S. air force in WW2, when four belonging to the 46th Pursuit Squadron managed to get airborne at Pearl Harbor to oppose the IJNAF - and a recoding as the earlier export equivalent Hawk 75A would do nicely to introduce a French Air Force fighter into the AH lineup for the very first time, a long-overdue development. Perhaps a Morane-Saulnier 406 for the French as well? The Russkie Polikarpov I-15 biplane will do nicely for our Red friends, with perhaps a Yak-3 for the Normandie-Niemen fans. Netherlanders and the Finns would surely enjoy the Fokker D.XXI, it's got to be the PZL P.11c for the Poles - and that'll do for my contribution.