So, I'd been doing a bit of shower thinking about a possible underground zone, and it hit me that one thing they could include in such a place would be the Feymarch. For those unfamiliar, the Feymarch is a location from Final Fantasy 4.
https://finalfantasy.fandom.com/wiki/Feymarch
The concept could be adapted in a number of ways. For instance, they could stick to the concept faithfully, and have it be a place were Eidolons/Primals can manifest without summoners. To justify that, perhaps it's exceptionally close to the planet's lifestream, even more so than the Antitower. Or, taking other inspiration from FF4, specifically the presence of Sylphs in a similar locale in the Underworld, it could be a place where we find the Source equivalent to Norvrandt's Fae Folk. Or they could take the etymology of Feymarch into consideration and, going back to the aforementioned "close to the lifestream" idea, make it a "land of the dead" sort of place. That would kind of double as a FFX reference with the Farplane as well, now that I think about it. Or it could be an Ancient facility dedicated to the study of arcane entities. There were cities other than Amaurot from that era, after all, and such a place could loop back to the mysterious "Sound" that led to the Final Days.