Well this is based from another game but the idea could be a good addition regardless:
Cosmetics that represent a player’s achivement, such as max forage, max exploration, max etc.
Another more “special” max cosmetics could be maxing a combo, such as max gathering skills (forage, fishing, chopping, mining, others), max artisan skills, etc.
If you (dev(s)) are actually interested in this idea you could ask the community itself for ideas for each skill cosmetic, given that the community is also interested in this.
(And of course, a set limit to how many cosmetics you can wear at a time (for example, 1 on the head, 1 on the body, 1 on the legs, or just 1 regardless of body part), after all, I dont think a player wearing every single cosmetic at the same time would be a pretty sight)