Mastery system - Trials to evaluate player' skill

The biggest problem with a system the only shows the the level of the player, is that you cant gauge the players actual skill level. Sure they have grinded for hours to get here, but that doesn't prove that they are actually good.
I suggest adding small trails around the map, as an optional quest, that will test the players skill level, regardless of if they are level 5 or level 50. These trails will come in different difficulties, and if you win you will have unlocked a mastery level in e.g. sword fighting.
Minor cosmetics could be unlocked, by having a high enough mastery in a skill, but the main goal would be to have it as a "badge of honor" showing other people, that you are not just another grinder, but also a skilled player.