Its likely the BLM/SMN casting gear. I swore I saw a black mage using it in the trailer during the Moogle fight.
That or its just bizarre vanity gear.

And my God not everything is a class teaser. The addition of a new class/job is a huge deal, and requires a ton of balancing. Before adding it to the game, they'd tease it in release photos or artwork, confirm it, then spend several months putting out info and showing it in a video. It would never just fall in our laps in a patch like this.