I'd still go by numbers produced in WWII for the various countries for adding planes, with some ordering based on what would be nice to fill out scenarios. The list below are the most-produced planes, for each country, that are not already in the game. I then shuffled and interleaved the order of countries based on a preference for which aircraft are most useful for scenarios. For example, the Japanese plane set is missing its major bomber (the G4M) and it's best and most-produced torpedo and divebombers (the B6N and the D4Y); and the Russian plane set could use one of its major attack planes (the Pe-2) and more representation for early and mid war (I-16, Yak-1, etc.).
He 111, 7300 -- for Battle of Britain
Pe-2, 11427 -- a major Soviet attack plane, 2nd only to the Il-2
G4M (Betty), 2446 -- the main Japanese bomber
D4Y Suisei (Judy), 2038 -- the most-produced Japanese divebomber (better and faster than the D3A)
B6N Tenzan (Jill), 1268 -- the most-produced Japanese torpedo/carrier bomber (upgrade of the B5N)
I-16, 9004 -- for early-war German/Russian scenarios
Yak-1, 8720 -- for early-war German/Russian scenarios
Ki-43 Hayabusa (Oscar), 5919 -- a much-used Japanse fighter in the earlier years
LaGG-3, 6258 -- for early-war German/Russian scenarios
Ki-21 (Sally), 2064 -- a much-used Japanese bomber
Yak-7, 6339 -- for mid-war German/Russian scenarios