B-25 -- has broadest allied use, widest time frame, and great variability (I WANT that strafing mod). Or, Russian Pe-2. German mediums already covered, and easier to shoot at anyway.
Mig -- broadens russian plane set, and low level usually all that matters here anyway. Or, another Yak. P-39 could die in many theaters with many skins, so also a good option.
T-34 76mm. The 85 would need serious perking, as it would routinely snack on standard panzers. Of course, once the 76mm modelled, the T-34 85 would be and easy upgrade, and give the GV player with perks burning holes in his pockets something else to buy. Or how about Werbelwind -- wasn't that the 4x20mm flakpanzer? It would bridge the m16 to Ostwind gap, and make bases that much harder to take!