LaGG-3 would be a nice addition, as would the Yak-1 and/or Yak-7. Il-4 or Pe-2 would also be good Soviet additions.
We are running out of the kind of unit that was used in multiple theaters. Some of the Italian stuff perhaps, C.200, CR.42 and SM.79-II were used in both the Mediterranean and against the Soviets. As the multi-theater units are almost all added then we ought to look to things that, while not multi-theater, still saw a lot of use. Things like the aforementioned Soviet stuff as well as Japanese units like the D4Y, B6N and Ki-45. Earlier versions of existing units, such as the B-17F, B-24D, Bf109E-7, Bf109G-6/AS, Fw190A-3, Ki-43-I, La-5, LaGG-3, Mosquito Mk II, Mosquito Mk IV, P-38F, P-38H, P-47C, P-47D-5, P-51A, Spitfire Mk II, Yak-1 and Yak-7 as well as things like the Lockheed Ventura or even the H8K2, PBY-5 and Short Sunderland.