In Final Fantasy IX Memoria is formed as a portal back to the original crystal because Terra died. According to official lore, when a planet’s crystal dies it returns to the cosmos. The same way souls return to the planet’s crystal, the planet’s collective soul (crystal) returns to the cosmos; I assume this is the original crystal you see in the ending of FFIX. Memoria is a portal that leads to the beginning of all time: the crystal. Around the crystal you see two streams of souls, red and blue, circling it (souls). The downside of this theory is though that Memoria is filled with memories of Gaia rather than Terra though… Guess Terra’s memories could have been eroded/mixed with Gaia’s.
I don’t necessarily think it is the intended course of events but I liked a theory I heard about FFIX. At the end of the game you go through Memoria and the further you get the further back in time you end up in. You get to a time where Terra and Gaia were shown merging, to a time Gaia was completely covered in water, then to a time where Gaia was just a big ball of lava…then you go so far back in time, there is nothing anymore, just a field blackness that you walk across. At the end of this path, you get to Crystal World, the beginning of the universe. Kuja is there wanting to destroy the Crystal, the source of all life. You fight him and at the end of the battle he casts Ultima which defeats your party and everything fades to black (later you wake up on the Hill of Despair, some kind of limbo dimension).
The theory I heard was that Kuja did shatter the crystal with his Ultima spell, but this did not end the world and instead was the “big bang” that really created the universe. All the planets have a crystal inside of them as is explained in the game, which could be shards of this original crystal.
I think that’s probably not true though, but a neat idea.
I wrote about this here, but only touched on the aspect of how all connect.
This would mean that Final Fantasy IX’s Cids, Chocobos, Moogles and Summons and the crystals are all formed from Final Fantasy IX’s world. Every memory the characters had, including all of the references (as Final Fantasy IX is rife with homages) informs the Final Fantasy universe of what worlds are to be contained in them (in much a similar way that Lightning and friends create a new world for them to live in).
But I glossed over the idea that Kuja succeeds in becoming a literal god over the Final Fantasy universe.
Then, could Kuja have become the god of Lightning Returns: Final Fantasy XIII, Bhunivelze? When Lightning recreates the universe, does she leave out Final Fantasy elements?