Xal'atath is a logical villain to utilize for TWW. Azshara is benched until Midnight, as she is better fit as the villain of an Elf-based expansion. Denathrius is too recent, and a Void Lord is too important to be the villain of TWW, which is just a set-up expansion.
I'm not surprised that Xal'atath was promoted from "tertiary character that only Shadow Priests interact with" to "box art main villain". There's barely any villains left for Blizzard to utilize. The Legion is dismantled, the Old Gods are """""""""""dead""""""""""""", and the Scourge is a meme. So who, if not Xal'atath, for TWW?
Well, I guess we could have another expansion about the Horde being the main villains.
