I think an important factor here is that it's the anniversary event key art. It's not going to be connected to the story, because this is gonna be served up to people still in A Realm Reborn; similarly, any content it's attached to will be released during the Rising, rather than any major content. (Incidentally, that second one also knocks out a contender I've seen put forward that you didn't: Asura, the Fanfest trial.)

So whoever this is, it can't be someone whose description would require dipping into spoiler tags.