Thanks to gsurfer for the inspiration!
The world of XV is the New World from Lightning Returns but far into the future.
This theory may not be so far off. In fact, this might have been the original intention. Final Fantasy XV’s logo featured the sleeping figure of the Goddess Etro. For those who’ve kept up with the decade-long development cycle of Final Fantasy Versus XIII, we know that what became Final Fantasy XV was originally a part of the Fabula Nova Crystallis universe.
Final Fantasy Versus XIII was a game planned for the PlayStation 3 as part of the Fabula Nova Crystallis: Final Fantasy series. It was intended to be a companion game to Final Fantasy XIII alongside the mobile game Final Fantasy Agito XIII, the three sharing from the same Fabula Nova Crystallis mythos. Bearing many originating themes carried on into its succeeding game, the theme of Versus XIII was “a fantasy based on reality”, and the game was to take place in a setting similar to contemporary Earth.
Similar, but not exactly ours. Can remember Final Fantasy XV’s tagline “A fantasy based on reality”? Then you’ll get a sense of the original plan.
The new world is the world created to replace the dying Nova Chrysalia in Lightning Returns: Final Fantasy XIII. The birth of a new world where Lightning would reunite with her sister, Serah, and all of her old friends, acts as Lightning’s main drive and the crux of her quest; her duty as the savior is to save the souls of the dying world and ensure their rebirth in the new world. The world is not named beyond “new world”.
The “new world”’s name is a task the inhabitants would act.
The new world resembles Earth with its continents mirroring those of the real world, and the town seen during the epilogue FMV features modern buildings, roads and cars, a modern train and a train station, and there is a plane in the sky that creates a contrail. Additionally, the souls of humanity appear to fly by celestial bodies strongly resembling Saturn and Mars, two planets that share the Earth’s solar system. The world has a moon and familiar plant life with grass, wheat and Flanders poppies.
The key word: “resembles”. It’s similar enough to where we can recognize many of the conveniences. This would also explain the existence of Astrals, which may have been based on fal’Cie.
Of course, Final Fantasy Versus XIII became Final Fantasy XV. Also, I’m not sure why Lightning would create summons to mirror the fal’Cie. Wouldn’t her intention be to create a world without their presence?