Transmogrification improvements will allow players to collect item appearances and save favorite outfits without sacrificing bag space.
This Diablo-esque system was teased at past BlizzCons, and in Wowhead's 6.2 Interivew, it was stated that the transmog system was on the front-burner, not the back-burner.
The transmog system was briefly mentioned on the Legion website, but not discussed in any interviews or presentations.
Artifact weapons can be transmogged to other weapon appearances, but other weapons cannot be transmogged to the Artifact appearance.
Some old transmogs, such as Mists of Pandaria capes that are no longer obtainable, may be transmoggable in the future. Other old legendaries that you can farm up, such as Warglaives, will not be moggable.