I would have gone with the Prototype Fal'Cie Adam since it's the semi optional boss in the future tech city. Not that I would want a stuck in a time loop boss fight.
There is so much to draw and compare to things we have seen when it comes to speculating about who or what made Solution Nine. Or how it came about. You have the Machi/Levin purple, the select few hexagonal and triangular doors seen in the dungeon that's probably connected to it giving off Allag vibes. The Tron and Cyberpunk feels. Though I tend to go with Chronopolis from Chrono Cross myself. Not quite the sanitized white futuristic look as cities akin to Neo Midgard, or Arkives from Star Ocean: Til the End of Time. The crate stacks do look the same shape as crates found in Azys Lla that the Garleans use, but with purple lines on the side.
I know many are tired of Ancients stuff but it would be interesting if this was as some have said the Nineth Solution our past self tried in protecting people from the Final Days or the sundering. It could also be parts of Ancient civilization built on by those who used it as shelter. Or something made by those those who found a way to shard hop from the 2nd Calamity which would be denizens of the 12th reflection. I do ascribe to the theory that we are probably dealing with some form of Ethar/Wakanda situation as the city is too high tech to not have been hidden. With Heritage Found being the public facade.
I just hope we aren't dealing with a Midgard/ Atlas from RWBY type situation where those above the lightning and or purple dome thing are the only ones that matter vs the ones below are treated as disposable.