In a group interview, Game Director Ion Hazzikostas recently commented on the intent to add previously removed cosmetic appearances, with the exception of very specific notables like Ashbringer. We uncovered several of these recipes in a previous PTR build, but now have actual items and appearances to go along with them, including not only items lost in the Wrath of the Lich King Naxxramas update, but also rewards from quests that were removed from the game during the Cataclysm world revamp.
Group Interview with Ion Hazzikostas
They want to add ways to get appearances that exist in the game that are no longer available / have never been available back into players’ hands. They want to hopefully get to a point where there are no remaining things in that backlog, other than ones that are very deliberately no longer available like Corrupted Ashbringer.
It is currently unknown how to unlock the Naxxramas appearances, but if it follows a similar pattern to the Zul’Gurub cosmetic collections, they are likely to remain encrypted on the 10.1.5 PTR, to be discovered once the patch hits live servers.