I'll come back to the rest later - it has interesting bits, looks very intriguing, and I would say nothing that fundamentally alters my current thoughts on what is to come...
(particularly with Emet narrating a descent into the Underworld/Aetherial Sea), but in any case...
This seems to be set with a Pandaemonium backdrop. It is clearer here:
Comparison to Pandaemonium from Dissidia (based on 2):
It does resemble the typical pre-raid fight staging area. Still think they're using the instance to give him or a concept vessel-summoned version of him a proper fight that a trial won't suffice for.