When it comes to theming, we get our most substantial information from the XBox News interview.
Here is the image for reference:
I often see that the "ambition of mankind" is interpreted as being a positive thing by people, mentioned alongside it being a brighter game. However, Miyazaki says this character represents both the darkness of the world
and the ambition of mankind. The dude is wearing someone's skin and standing in a place that is on fire holding a sword. Given all of this, I'd say the ambition of mankind in Elden Ring is not a good thing (not that it might not have a duality or other facets, I'm sure it will be more nuanced than that), and that all of this gives me a sense of mankind conquering and destroying.
So to bring it back to the discussion about the barrier Omni mentioned and the grand mystery of the shattering of the Elden Ring,
I am tempted to feel that there is a decent possibility that mankind lead some sort of rebellion against the gods/ruling forces of the world, or just tried to elevate themselves above their station, and the Elden Ring was shattered by mankind (or a faction of mankind). This goes back to things like the apple in Eden, Prometheus stealing fire (which leads to Pandora's box), the Tower of Babel, etc.
It is speculation, but with the information we have, I don't think it is completely out of bounds.