From a scenario perspective (generally picking a country's highest-production aircraft not already in the game), the best planes to add would be as follows.
He 111 (Important for Battle of Britain)
Yak 1 (For Eastern Front)
D4Y (Most-produced Japanese dive bomber, taking over for early-war D3A)
B6N (Most-produced Japanese torpedo bomber, taking over for the early-war B5N)
Pe-2 (Most-produced Soviet attack plane other than Il-2, for Eastern Front)
Ki-43
TBD (not as highly produced as TBM, but important for Battle of Coral Sea)
Aircraft production in WWII
---- US ----
For the US, here are the production fighters, with numbers produced listed (from America's Hundred Thousand):
P-47, 15683
P-51, 15486
P-40, 13143
F6F, 12275
F4U, 11514
P-38, 10037
P-39, 9529
F4F/FM, 7905
For bombers (from
https://www.airforcehistory.hq.af.mil/PopTopics/AAFaircraft.htm and others):
B-24, 18190
B-17, 12692
B-25, 9186
A-20, 7385
B-26, 5157
TBM, 9837
SB2C, 7140 <--- not yet in AH
SBD, 5936
---- JAPAN ----
IJN aircraft from
http://www.combinedfleet.com/ijna/ijnaf.htm and from
http://en.wikipedia.org/wiki/List_of_aircraft_of_Japan%2C_World_War_IIA6M Zero, 10449
Ki-43 Hayabusa (Oscar), 5919
Ki-84 Hayate (Frank), 3449
Ki-61 Hien (Tony), 3159
Ki-45 Toryu (Nick), 1701
Ki-44 Shoki (Tojo), 1225
A5M (Claude), 1094
J2M Raiden (Jack), 476
N1K Shiden (George), 415
D4Y Suisei (Judy), 2038
D3A Aichi (Val), 1495
B6N Tenzan (Jill), 1268
B5N (Kate), 1149
B7A Ryusei (Grace), 114
G4M (Betty), 2446
Ki-21 (Sally), 2064
P1Y Ginga (Frances), 1098
G3M (Nell), 1048
Ki-67 Hiryu (Peggy), 767
---- RUSSIA ----
From
http://en.wikipedia.org/wiki/List_of_military_aircraft_of_the_Soviet_Union_and_the_CIShttp://en.wikipedia.org/wiki/List_of_aircraft_of_Russia%2C_World_War_IIhttp://english.pravda.ru/main/18/90/363/9941_roosevelt.htmlYak-9, 16769
La-5, 9920
I-16, 9004
Yak-1, 8720
Yak-7, 6339
LaGG-3, 6258
La-7, 5753
P-39, 5007 (from US)
Yak-3, 4848
MiG-3, 3120
Hurricane, 2952 (from UK)
P-63, 2421 (from US)
Spitfire, 1331 (from UK)
Il-2, 29937
Pe-2, 11427
SB-2, 6656
Il-4, 5256
A-20, 2700 (from US)
Tu-2, 2527
DB-3, 1528
---- UK ----
From Wikipedia entries starting from
http://en.wikipedia.org/wiki/List_of_aircraft_of_the_RAF and using
http://www.rafweb.org/aircraftA-D.htm to judge which are WWII aircraft.
Spitfire, 20351
Hurricane, 14000
Typhoon, 3330
Seafire, 2334
Tempest, 1702
Defiant, 1064
Mosquito, 7781
Beaufighter, 5928
Beaufort, 2080
Wellington, 11461
Lancaster, 7377
Halifax, 6176
Blenheim, 4422
Stirling, 2383
Whitley, 1737
Hampden, 1430
Martin Baltimore, 1575 (from US)
Douglas Boston
Brewster Buffalo
B-17
Douglas Havoc
Curtiss Kittyhawk
B-24
Martin Marauder
Martin Maryland
B-25
Curtis Mohawk
P-51
P-47
Vultee Vengence
Lockheed Ventura
---- Germany ----
http://en.wikipedia.org/wiki/German_aircraft_production_during_WW2, but using individual entries for aircraft production.
Bf 109, 35000
FW 190, 20010
Bf 110, 15000
Me 262, 1430
Ju 88, 15000
He 111, 7300
Do 217, 1366
He 177, 1146
Ju 87, 6000