keep the challenge system the way it currently is, but at the end of each tour, (or instead of every tour do it quarterly), have a koth type event amongst the top 5 and declare the winner the top fighter pilot of the month (quarter).
then blow up the list!
re-scatter the player order of the list putting them in different positions, (again leaving the top three spots empty) forcing everyone once again to fight their way to the top. this will prevent the list being completely topped by only the cream of the crop becoming stagnant and making it a boring and uninteresting side note of the game.
keep it changing and challenging to all new and old, will help keep it alive vibrant and interesting to all.
just puking out my thoughts for the masses to consume.....