The aircraft or GVs of each release should complement each other, so that they can be used in scenarios, TOD, the CT, etc.
So here are several possible 3, 4, and 5 plane options I'd consider:
1. Early-Mid War Pacific
Ki-43 Hayabusa / Oscar
P-38E (or F)
P-39N
Mitsubishi G4M2 Betty
North American B-25C
2. Russian Front
Petlyakov Pe-2
Mig-3, Yak-1, or Yak-3
Ju-87D-5 (w/ 2 x 37mm)
Heinkel He-111H-3
3. Late War Pacific
Boeing B-29 Superfortress
Nakajima Ki-44 Shoki / Tojo
Kawasaki Ki-45 Toryu / Nick
4. France 1940 & Early Mediterranean Theater
Bristol Blenheim IV
Savoia-Marchetti S.M. 79 Sparviero
Junkers Ju-52 Tante Ju
Dewoitine D. 520
MRPLUTO