I have a theory that Mira is the remnants of the Bionis and Mechonis. I know that X is a spiritual successor and not a sequel, but still, hear me out.
1.) The continents are very similar to locations on the two titans such as Primordia being Bionis leg and the surrounding areas, Noctilum being Makna and Satorl, Oblivia being the ruins of Sword valley and Fallen arm, and nothing is able to grow due to the massive machines polluting the land. Sylvalum is Eryth Sea and Vallek mountain I guess, It is the only one that doesn't really fit in, while Cauldros is the ruins of Mechonis. It also has been thousands, perhaps millions of years since the defeat of Zanza, so of course the land has changed quite a bit.
2.) Two species return from Xenoblade, being Nopon and Telethia. It is even hinted that they know something about ancient history, as they (The Nopon) call the humans hom-hom, while Tatsu says it is the heropon's role to slay evil gods, a reference to Zanza and Riki. The Telethia are the divine servants of Zanza, and one must have survived and gained an incredible amount of power over the years, becoming Telethia, The Endbringer. The telethia is also described as a divine protector of Mira, similar to how they defended the Book is, but also bears the title of Endbringer, like how they attempted to bring about the end of all life on Bionis. With Zanza dead, however, it has no one the return the ether it collected to, so it reached it limit to its power, causing it to be so strong.
3.) The other races have all been wiped out, sadly. Perhaps it was gradual, such a decreasing birthrate eventually leading to the extinction of several races, or perhaps, it was cataclysmic events such as a flood in the Machina Village, or an avalanche in Valak mountain, that wiped out these races that still where recovering their rather small populations from the recent war against the mechon. Whatever the case, only the Nopon and Telethia endured and remained true to their roots, while other races evolved, such as the Gogols evolving into Simius.
4.) One of the citizens of NLA says a Man-on once told him the space around Mira was small. This could mean that the experiment Meyneth and Zanza lead did not destroy the Earth as they thought but rather took them away from it, into a new dimension. It is already canonical that there are different dimensions, as Luxaar states that the Sumaarians arrived from another plane.And while on its two year voyage, the white whale somehow traveled into this separate dimension, with the Ganglion right behind.
I know that this is probably not true, but it was interesting to write about. Once again, I am aware that X is a spiritual successor, not a sequel, but it was just something I wanted to share with all of you.