Better than a "Walled-in" island, just limit the A/C type at the forward fields: one has IJ stuff, the other RAF, a third USAAF, etc.
You could have say four aircraft "groupings", along the lines of:
Group 1:
Spit
Typhoon
Group 2:
109
190
c.202
c.205
Group 3:
P38
P47
P51
F4U
Group 4:
N1K
A6M
Yak-9
La5
(Ok, group 4 is temporary until they add the Il-2, the La7, the Ki-61, the Ki-45, Ki-100, etc.)
Anyway, forward bases (one RW) have only one group available. Medium bases have two, and Large bases have all. That way, fans of a specific plane won't be left too far behind (they can always ferry aircraft if they're desperate), battles will be more spread out, and the furballing masses will be encouraged to try other rides.