Germans lack a mid-war bomber, candidates being Do217, He177 and Ju188.
Italians lack an entry fighter, C.200 or CR.42 and lack a bomber, SM.79-II or CANT Z.1007.
Russians lack fighter coverage for 1942 and most of 1943, LaGG-3, La-5, MiG-3, Yak-1 and/or Yak-7 and they lack a bomber, Il-4, Pe-2 or Tu-2.
British lack an early war bomber and a medium bomber, Wellington being by far the obvious choice.
Japanese lack later war carrier strike planes.
USA is light on early war carrier strike planes and late war carrier strike planes.
French lack everything.

Then there is the whole bunch of stuff that isn't needed, but would be fun and/or useful.