When I read the hypothetical post regarding the guild achievement and Award system, I thought that Blizzard's example of We Are Legendary granting the Dark Phoenix sounded like a solid idea!
Not too far back people started bleeting about how upset they were that the Dark Phoenix was a guild reward, bringing up old TRH complaints. If the Dark Phoenix was an unlockable reward from collecting every single not-removed legendary, that could very well make it about as rare as the Ashes of Al'ar given most legendaries are based heavily in RNG, it'd be like farming for the ashes six times over.
That is, unless, some clever player rallies together a wielder of each legendary, it would still take some time to build up Guild rep to buy it.
Also, just an idea given we have a Dark Phoenix Hatchling...
A tiered achivement in that aquiring ONE legendary weapon unlocks the Hatchling, collecting them all unlocks the mount.