I was just reading the Supply and Demand thread and it gave me this idea. It's not flushed out and is probably full of holes but here's the premise:
As planes, particularly low ENY planes, increase in useage in the MA's one of two things happens; either they begin to incur a perk price, or they continue to lessen in ENY value for perk accumulation. This could apply across the plane-set but I see the biggest impact on the "best" planes. I'd guess lowering ENY would be more palatable than imposing a perk price so I'll follow that idea for now.
All of this could operate on a mathematical formula. It would probably have to be based on country numbers rather than arena wide so that no country would be at a disadvantage. Similar to lowering ENY for high use planes the formula could increase ENY for low use planes.
So lets just say for example, the median ENY in operation in the arena at any given time is 10. There are 300 people in the arena evenly split between countries. As a country drops below the ENY 10 threshold overall, the lower ENY planes are adjusted down and the higher ENY planes are adjusted up.
This might keep people from upping the next Spit XVI or La7 if the ENY drops to 2-3 and encourage them to take up a 190 or a P-47 as the ENY on these rises.
I'd like to see it apply plane specific or at least airframe specific but I haven't thought far enough yet to figure out how that might work. Maybe there's a way to drill it down from overall to airframe to specific model with each having some influence over it's current ENY value. It wouldn't be much different than the currently fluctuating perk price values so I'm guessing some sort of implementation might be based on that.
Essentially, this turns ENY into a supply and demand proposition rather than an arbitrary number.