But if ENY was a floating value, those planes would constantly be in flux.
I honestly don't think there is much that can or should be done about the mix of planes. Really the only thing that would have an impact of gameplay would be HTC responding to the playerbase's desire to shift the focus of the game away from aerial combat more toward the landgrab aspect by adding a larger variety of GVs. With the exception of the Yak-3, we already have every plane that saw any sort of action in WW2 but also has the performance to warrant being used in place of the current big 4. For that matter, I can't think of a plane other than the Yak that would even crack the top 10.
There are still a lot of holes to fill as far as early / mid-war planeset for the Soviet and Japanese planeset, but the only really large gap left is the GVs. Still missing all kinds of things that would see use, and some that might even change the way the game is played.