the gist is to add a cosmetic badge to your inventory once you complete x which you can then equip to your profile (up to a maximum of 6)
maybe throw in a few easter egg badges as well
examples:
Spoiler

Identified! - Identify account

Performer - 1000 Carnival wins

Diver - Find 1000 loot while diving

Explorer - Decipher 500 mystery maps

Juggernaut - Kill 50 players in a row without healing

Spartan - Buy the Spartan helm from Nexus 2018

Santa's Accomplice - Send 500 or more gifts during Christmas

Easter 2018 - Completed egg collection

Halloween 2018 - Collected x amount of candy

Partners Furever - Summon your partner while morphed

Esteemed Host - Have 50+ people in your guild house at once
in-game display
Pros
+ gives graphics staff more creative liberties outside of making hats
+ good way to showcase achievements
+ commemorates discontinued events
+ incentive to do less popular activities
+ collect em all
Cons
- requires work to implement