5.0 spoilers.

Aside from other things mentioned in this thread, I also got IX feels from the fight with Hades.

In IX, you fight Hades in an area constructed of memories. He's found in the ocean room.

In XIV, you also fight Hades in a memory/ocean zone.

I thought his design was similar to Oeilvert too:





"When Zidane and his party enter Oeilvert, they find it was built by the Terrans and holds devices that record Terran history. The holographs strewn across the ruins depict a Terran civilization that was advanced and inquisitive about its world, yet ultimately dying." - from here: https://finalfantasy.fandom.com/wiki/Oeilvert

Also, you learn about the construction of warships like IX's Invincible in Oeilvert. That warship is a reference to an airship from III, but IX's is notable for absorbing people's souls.

One of XIV!Hades' battle themes is fittingly titled "Invincible."