I find the mythos of FFXIII really interesting, not just because who doesn't like mythos building, but because it's why I think the sequels are so scattered.
Mythology was obscurely referenced in the first game but only if you worked hard decoding the analects. XIII-2 worked with Etros, and LR finally tells you the entire mythology. But most of us already knew it before XIII-2.
This was released shortly before XIII-2. If you want to tl;dw it, there's basically a hierarchy of godhood at work.
Mwynn and Maker=Bhunibelvze
Then Etros, Lindzei, and Pulse
Then all the fal'cie and humanity, with humanity having the power of free will and fal'cie not.
FFXIII, actually all the games from the Fabula Novas Crystalis share the same mythology, just interrupted differently. In fact, from the way I interrupted how they handled it, it was actually the mythos built first, then handed to different teams (Agito/Type-0 and a Versus/FFXV) and then they built the story from there, allowing creators to put their own spin on it.
I could be wrong but it makes a lot of sense. FXIII-2 had time travel, not because the first game didn't disallow time travel, but because there's nothing in the
mythology that didn't allow time travel. It was the mythology, not the OG that was the backbone, it was the predetermine background. So sequels could stray as far as they wanted from the first game.
XIII's OG actually handles very little of the mythology. The meat and potatoes were focused on a small part of the mythology, fal'cie and humans. They mythology was there, but in a very background sort of way: a lot of how things happened and why it happened could be explained by the mythology (Etros saving the people in the end), but very little of that is explained in game.
It wasn't important to understand to understand these things to understand the basic story, which was a sort of post-Iraq war criticism of paranoia/ self determination/ screw fate story. The sequels did away with the first theme, and kept the second and third theme of defying fate and humanity making its own way.
That stuff was all there, but only if you dug deep. I actually like they way they handled it, it didn't bog you down with too much information in an already confusing story, but for those that were interested, if you took the time and energy to search the game, do all the hunting quests, you could read the analects, it told you all sorts of things.
I barely knew there was more than one god in the first playthrough. But by the time I hit gamefaqs, most people not only knew there were three active gods, but I read someone accurately guess there was a fourth god that we knew next to nothing about (and considering how the analects were written, this was difficult to figure out).
Anyway, the games actually tackle different rungs of the mythology. XIII you fought against the fal'cie (Etros and all were present, but they weren't the primary antagonists). XIII-2 has you working with Etros. By the time you hit Lightning Returns you are facing off the entirety of the mythology and Bhunibelze, and because the entire theme is fighting fate, you've got to take on Bhuni and hit the reset button on the mythos and destroying the need for all the gods with the exception of maybe Etros.
It hits a weird split between fans. People who know about the mythology were more interested in seeing the story progress god wise rather than just having a sequel in XIII, and were happy to see when we got more mythology stuff about Etros, creation, and Bhuni. But people who didn't read into the mythology, either by youtube or reading analects were completely lost and annoyed by the sudden shift in change.
It's especially apparent when reading about the creation in the last game. For most of us, we already knew shit about Bhuni and chaos, so it was all rehashing, but at least they were talking about relevant stuff. The people that didn't were fucking confused and had no idea where it was coming from.
Honestly, looking back, they could have done two different things to help subdue the rift. Either make more of the mythology accessible in game in XIII and XIII-2, or stay somewhat outside of the realm of the creation story and focus on things that are more inline with XIII's story.
Honestly, sometimes I wonder if the story was just there in the sequels for the developers to play with different mechanics. I thought time travel was fun in XIII-2, but not ambitious enough, and Lightning Returns open world, time management and use of time as a resource was too ambitious for a a one year, total renovation development cycle and Toriyama's terrible directing/already confusing story would allow.
Oh well.