Just had an idea.
Add perks per hour as a score category.
Why?
We generally want to promote usage of the lesser flown planes, but people too often fly the same planes because those planes help them get the lowest score. If perks/hour were added as a category, people would have a SCORE incentive to fly higher ENY planes. And far more people are driven by score than those that are driven by appreciation of the entire planeset.
It's easy. The server knows how much time a player has logged, and how many perks they have earned in any given time block. Increment time to hours, and divide total perks earned by hours played. We have the same thing currently with kills/hour.
Don't factor in perks spent/lost. Makes things too complicated and would only encourage even more timid flying in perk planes. Just simply how many they have earned/hour, and obviously fighter perks would factor into fighter score, vehicle perks...you get the idea.
I'm not saying this would drastically increase the variety of planes in the air...but it's a start.