Originally known as Final Fantasy Verses XIII, Final Fantasy XV seems to be holding over some of material inherited from the original designs of the project. Or Final Fantasy XIII, Final Fantasy Verses XIII and Final Fantasy Agito (later called Type-0) were all part of the Fabula Nova Crystallis, a trilogy of games focusing on different characters, but connected in some, at the time, unexplained way.
Final Fantasy XIII trilogy director Motomu Toriyama explained to IGN in an interview that there are still connections between XIII and XV, despite the name change. One of these connections was the goddess Etro. Lightning serves the goddess in XIII and a clear reference is made to her in the debut trailer of XV.
“You must remember, it was a long time ago that we named those three games: XIII, as well as Versus and Agito which we now call Type-0. Obviously those three projects were going to be developed against the backdrop of this Crystal trilogy, or Fabula Nova Crystallis. But each project has a different set of characters and they’re set in different universes and worlds.
“When Etro is mentioned in the context of XIII and XV, we’re referring to the same goddess of Death. It’s the same goddess, but how she’s featured, how she appears and how she’s involved with the story is completely different. They’re different stories and different projects. So while it’s the same character, her involvement is very different.”