Japan
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D4Y - LAte war single engine bomber
G4M - "Betty", main IJN level bomber
J2M - IJN interceptor, Malaya/Phillipines
Ki-27 - Battle of Khalkhyn Gol with the I-16
Ki-43 - Main IJAAF Fighter of the war
Ki-44 - IJAAF Oil Fields scenario
Ki-45 - Japanese Heavy Fighter - New Guinea scenarios
Russia
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I-15 - as Ki-27, plus Spanish Civil War
LaGG-3 - most modern Soviet fighter in 1941
Mig-3 - Early war, Russia '41
Pe-2 - Eastern Front, Russia vs Finland
SB-2 - Russian early war bomber - Khalkhyn Gol or Russia '41
Tu-2 - Russian late war bomber
Yak-3 - Late war competitive bird
UK
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Beaufighter - Used pretty much in all theaters of the war
US
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P-63 - Late war competitive fighter (ish... the 37mm isn't all that great)
P-36 / Hawk 75 - Early Pacific (Dutch vs Japan) , Battle of France (1940), Finland vs Russia
Italy
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CR.42 - Early War Med - Italy vs. UK
G.50 - Same
MC.200 - Same (and possibly easier to include than the G.50?)
SM.79 - Italian bomber for scenarios
France
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D.520 - Battle of France (1940)
Germany
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Bf 109E-3 - Battle of France (1940) (unless our E-4 isn't substantially different)
He-111 - BoB, Battle of France
Ground Vehicles
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Some sort of Self Propelled Howitzer - mostly because it would be awesome
Vanilla M4 Sherman (perhaps a 75mm and a 76mm varient?) - North Africa, Italy, Western Front
Panzer III - Battles of France (37mm), North Africa, early Eastern Front (37mm and 50mm)
ISU-122 or ISU-152 - Eastern Front (and would be fun in the MA)
StUG-III
StuG-IV or Jagdpanzer IV or Hetzer
M18 (only problem is an open topped turret)
M3 GMC (M3 with a 75mm - SPH and AT, easy mod to what we have)