10th Anniversary Amano key art [6.5+ Spoilers?]

[Theodric]

I'd be very surprised if it wasn't just Asura, who is already playing a part in marketing and teasers. It fits the golden aesthetics and 'gold' seems to be a key colour theme for the expansion in general. She'll likely be a trial or dungeon boss at a pivotal moment in the story, presumably if and when the player character reaches the fabled 'Golden City'.

[Cleretic]

I'd be very surprised if it wasn't just Asura, who is already playing a part in marketing and teasers. It fits the golden aesthetics and 'gold' seems to be a key colour theme for the expansion in general. She'll likely be a trial or dungeon boss at a pivotal moment in the story, presumably if and when the player character reaches the fabled 'Golden City'.

Again: she's gonna sit out the anniversary events thanks to being a Fanfest-exclusive thing, so it'd be very weird if it was her. Not to mention the fact that I've seen pictures of the Golden Araya arena, and part of it very clearly shows it as adjacent to Radz-at-Han. ...which also knocks out her being an event thing, because they wouldn't tie an anniversary event to an Endwalker zone.

Also it doesn't actually look that much like her, but it doesn't actually look that much like anyone being put in contention, so that's not as much of a breaker as I feel like it should be.

[Lyth]

Again: she's gonna sit out the anniversary events thanks to being a Fanfest-exclusive thing, so it'd be very weird if it was her. Not to mention the fact that I've seen pictures of the Golden Araya arena, and part of it very clearly shows it as adjacent to Radz-at-Han. ...which also knocks out her being an event thing, because they wouldn't tie an anniversary event to an Endwalker zone.

Also it doesn't actually look that much like her, but it doesn't actually look that much like anyone being put in contention, so that's not as much of a breaker as I feel like it should be.

There's pre-existing artwork by Amano of FFIV's Asura, which depicts her as wielding multiple swords and shows her different faces. The event boss' design was based off of that image. If the arena is tied to Radz, as you suggested, then perhaps the fight will serve as explanation around the 'old gods' that Matsya referenced in the Hannish prayer from the 6.0 MSQ.

The image linked in the OP looks completely different to his art of Asura. I'm leaning towards the earlier idea of there being a 'Will of the Star' that predated both Hydaelyn and Zodiark, but we'll see. Perhaps there will be more hints at the next Rising.