So if the title isn't enough of a suggestion...
Create some new and unique cosmetic items that can be earned as a reward for certain achievements. I cannot stress 'new and unique' enough, they have to be worthy of the achievement, not like this Crown of Fire junk.
Examples:
- Add a new Tier to each Monster Manual creature type. Award a Hat, the design of which is based on the type of creature.
- A Hat to go with Epic Completionist. With One feather. Each repeat completion, you get a new hat with an extra feather.
- An outfit for Heroic Completionist. Style to match the Epic Completionist Hat (they are a set). Increasing number of medals on the chest for each iteration of Heroic Completionist.
- Raid Master. A hat for completing a Raid on Elite, at level. Different hat for each Raid.
I'm sure there are plenty of other opportunities to expand on this idea.
Oh yeah, convert the founders helm to a cosmetic item too.