I like this idea; however, I'm not positive if the Completionist's Cape is compatible with Emps-World. That can easily be found out.
As a slight addition, perhaps these could be the requirements for a Quest Cape for those who aren't maxed; maxed players get both the Quest Cape and Completionist's Cape.
Whenever new content is added, such as a new (non-seasonal) quest, a new mini-game, or some new rewards, the players need to achieve whatever the new requirements are in order to wear the cape again -- any worn capes are unequipped.
This seems like a pretty hefty update, though.