The Cosmology of FFXIV had thinking about the Previous Final Fantasies

[Kaliesto]

Every Final Fantasy Universe has a reflection of every other world seen in every final fantasy game (with a couple of exceptions), this was somewhat shown in Final Fantasy IV - The After Years. How these alternate worlds operated with different cosmic rules is unknown (things like Magic, etc).

Every Final Fantasy Universe has a reflection of the original Character from his/her original universe that is later born into alternate universes (The Crystals doing perhaps)? This goes with point number 1 up above (and somewhat referencing Shadowbringers story events).

Some Monsters with a strong soul possibly have reflections in other universes of Final Fantasy, evidence of this is seen in the FF1 Bonus Dungeons, and FFIV-TAY. I'm going by what we already now know in Shadowbringers. The question is whether their true self has to die first to become multiple reflections spread throughout the cosmology.

Some Characters are capable of dimensional Hopping into another universe, but they do not retain these memories when they return to their original universe. There is a cosmological rule in play that prevents cross interference of other worlds. The Void seems to act as a barrier between different universes, and it becomes further complicated that every Void seen in Final Fantasy is not always clear-cut (See Cloud of Darkness of both FFIII and FFXIV which are two separate beings). It suggests that beings like Cloud of Darkness that plays some kind of major role in the void are multi-dimensional in nature, but not exactly the exact same being.

The Crystals seem to play more of a major role in the cosmology than what is being led on to believe, they seem to operate more universally and multi-dimensional than just sustaining a planet. The crystals seem sentient and can hold memories (evidence of this in FF1 CG movie, and the bonus dungeons). The Crystals didn't show their true power until FF9.

FFXIV story events and Special events that has happened over the years keeps screaming at me that that all the worlds of FF are loosely connected, but have rather strange rules.

Now this is just fun and dumb theory, but I know deep down Square Enix isn't thinking this far ahead. It would be cool though that the entire cosmology of Final Fantasy was operating under some vague umbrella yet to be revealed (unless Dissidia and Dissidia-NT is a glimpse of that).


EDIT:

I just realized that in FF9 that Necron was summoned under similar circumstances (under complete stress and Fear) when compared to the primals.

[Enkidoh]

The thing to remember is the older FF games were never designed to be part of a connected universe but merely reuse certain ideas and concepts (the very first FF was virtually a Japanese attempt at Dungeons and Dragons), a character named Cid being the main example of a shared idea who appears in every game but that is specifically a different character in every title.

But later titles did start to strongly hint that each title is part of a larger multiverse very loosely connected by the Void (NOT FFXIV's version, the FFV Void, also called the Dimensional Rift, which is a nothingness between universes that ties each FF world together vaguely), which was used commonly as the narrative device for explaining how certain characters from one title could show up in another, Gilgamesh from FFV being the supreme example of that. And if you take the Dissidia games as being canon to the main story rather than just a spin-off, this basically rings true.

FFXIV is like FFIX in that it celebrates and references every FF game that came before (and the ones that came after too!), but it's still it's own independent world - the development team have outright stated this.

Hydaelyn's Shards are not the previous 13 FF game worlds, they are literal copies of the 'Source' Hydaelyn where things turned out slightly differently (and are 'rejoined' when a Calamity occurs on the Source). The 'Shards are the other FF game worlds' theory was utterly debunked ages ago I'm sorry to say - the player's character is not a reincarnation of Cloud or Cecil, they are an independent character specific to FFXIV only.

[Berteaux_Braumegain]

To add onto that, there are only fourteen shards and we've met Noctis. Furthermore, Lightning has appeared from her world just fine and gone back home, yet the 13th Shard is the Void and inhospitable to life.