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).
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.
This would mean that Final Fantasy IX’s Cids, Chocobos, Moogles, etc. form from Final Fantasy IX’s world. Every memory, including all of the references, informs the Final Fantasy universe of what worlds are to be. Think in a similar way Lightning and friends created a new world for them to live in. Final Fantasy IX is rife with homages. It bypasses the sad ending of this article which states.
The physical world of Gaia still exists, but its crystal has been replaced with Terra’s; the crystal that was so close to death that the Terrans tried to fuse their planet with Gaia. That crystal is still about to die. None of the main cast seems to realize this and it’s unlikely there is anything that could be done about it even if they did.
Once Terra’s crystal dies inside of Gaia, it won’t be turned into a new planet. Kuja destroyed the Original Crystal that spawns all life with his Ultima spell. The crystal that handles the soul cycle for planets is gone. The world of Gaia will become a planet without life and every other planet will suffer the same fate until the universe becomes nothing more than an empty, lifeless husk.
In the infinite combinations of created, Final Fantasy IX’s world becomes one with a fully formed crystal. At least, that’s the hope.