ETO
Ju-87B-1
Do 17Z-2
P.Z.L. P.11
P.Z.L. P.37 Los B
P..Z.L. P.23b
Ca 310 Libeccio
Gloster Gladiator Mk II
Gloster Gauntlet
Fokker G.1A
Morane-Saulnier MS.406
He 111H-2
Fiat BR.20M
Beaufighter Mk I
Blenheim Mk IV
Hampden Mk I/TB.Mk I
Westland Whirlwind
SM.81 Pipistrello
Ju 52
Martin Maryland Mk II
Fairey Fulmar Mk I
Fiat CR.32
SM.79
Breguet Br.693
CANT Z.1007
Dewoitine D.520 (with both French and Regia Aeronautica skins)
Reggiane Re 2001 CN2
G-55/1 Centauro
Polikarpov I-15
Pe-2
Yak-1
MiG-3
LaGG-3
Yak-7
Lisunov Li-2 (w/ordnance option of VUS-1 dorsal turret)
Whitworth Whitley Mk V
Halifax Mk II
Vickers Wellington Mk.XIV
Stirling B.Mk III
Martin Maryland
Martin Baltimore
That is one weird list!
While having lot of planes that would definitely fill gaps like you said, there most certainly are many that wouldn't...
Gloster Gauntlet is a good example, what gap would it fill? Only action that it saw in the war was limited ground support in RAF colours against Italians. Even Finnish Air Force, desperate for combat aircraft, didn't use it in combat except brief experiments as a night fighter (no kills).
Then you have a Reggiane Re 2001 CN2 on the list which was a night fighter version of the fighter with only 74 built and at the same time missing the most important Italian fighter, the C.200. Again I really don't see exactly which gap Re 2001 CN2 would fill? Also why CR.32 and no CR.42? And so on...
These (for example) I certainly agree on:
Beaufighter
Ju-87B-1
Polikarpov I-15
3Pe-2
Yak-1
MiG-3
LaGG-3
Yak-7
Vickers Wellington
Gloster Gladiator Mk II
SM.79
CANT Z.1007