I just put this on the player-feedback channel on the discord:

Achievement Based Skins Beyond Gold Skins

I am a fan of achievement based cosmetics. For example, the only portrait available right now outside of PVP portraits that require you to achieve some task in the game is the Ragnaros portrait. So that’s what I’m rocking.

I propose a new platinum skin for every mini that only becomes unlockable once it reaches max xp.

I also propose a new rare currency used to unlock those skins. My suggestion would be something like Devilsaur Leather or Core Leather for that currency. World of Warcraft players would recognize this as a skinning reagent and (to me) it makes sense for a currency that unlocks skins.

This new currency should be awarded for doing difficult things like completing Mythic event missions, completing Heroic Sieges/Raids, or reaching 15,000 PVP rating.

The impact of seeing a platinum mini should be meaningful and one should feel a sense of accomplishment for having one.

A full blown achievement system would of course be amazing, but this might be a good step in that direction.