Apache,
Personally, I've always thought that "manufacturing zones" of a sort would be a nice implementation. ie: certain zones produced particular plane(s) which would then for the most part limit those to a general area. This could be done by using a number of standards, such as "era", "country type", "plane type", etc..
ie: perhaps a northern quadrant would manufacture British planes, and thus enemies wanting to fly British iron would be forced to implement attack/captures on the manufacturing facility(s) which produced that type.
Just a thought, but this would allow ALL planes in the arena to still be flown, add yet another dimension of "strategy" to the game, and I would think would be VERY simple to implement in the program code.
Limiting countries to various plane sets is another option, but too Draconian for me.
Regards, Oddball
Cactus Air Force