Good to have you back, Spock!
I think you're right on the money but also don't put aside your esoteric textbooks just yet! Nomura seems to be deep in his Qabalah era (Sephiroth's namesake, ofc). In Hermetic Qabalah, there are four worlds that the individual "ascends" to return back to the Divine/Spirit or whatever you wanna call it.
- The World of Assiah = material reality (probably what we already know of as "Reality" i.e. Realm of Light, Realm of Darkness, In-Between etc etc, also the world of the Body)
- The World of Yetzirah = the astral plane, or the "formative" world where imagination/"fiction" reigns supreme and "reality" is malleable. In the Qabalistic texts that I'm personally familiar with, dreams are a sort of window into this world; some also associate it with the Underworld, or the afterlife. This is the world of the "Soul," and is 100% where we're headed in KH (also cheeky connection to the Final World's relationship with Hades' literal Underworld). There's also something known as the "etheric body" and the "etheric world" which exists on the lower levels of the astral world, and sort of connects the Body with the Soul--it's the center of our emotions/emotional body. Safe to say that's probably the "Heart." (Thinking of that quote in the KH4 trailer: "The heart resides within the soul, which in turn is guided to its rightful place.") Also, worth nothing that the first Sephiroth (or "sphere") within the astral world that serves as the gateway into it is associated with the Moon (Kingdom Hearts?), called the "Gate of Yesod." Also from a certain perspective, the astral world "creates" the physical world by imagining it into existence -- there's our "layering" concept
- The World of Briah = the world of the Mind/Will, inner illumination, higher intuition, archetypes etc. -- can't think of any direct analogues in KH per se beyond occasional references to the mind and the will, but I have a hunch this is sort of what Kingdom Hearts itself represents? Or maybe something to look out for in the Lost Masters Saga
- The World of Atziluth = the divine world, the world of Spirit, ultimate reality etc etc. -- this is probably what lies beyond Kingdom Hearts. Also, something called the "Abyss" (or "Da'ath") separates the World of Briah from the World of Atziluth, and has to be crossed to get from one to the other. We've definitely heard of the Abyss in KH, "darkness within darkness" and all that. So like, Atziluth being the ultimate source of Light beyond the Darkness, or something.
Idk, thought the parallels were pretty cool. I was really into this stuff a couple years ago when MoM came out and then we got that KH4 trailer and I was like, this is totally where Nomura's drawing inspiration. I felt so vindicated when ML literally dropped the phrase "the Astral Plane." Of course, esoteric lore/Qabalah references are scattered everywhere in the JRPG world so it's not altogether shocking, lol
EDIT:
Oh, I should add that in Qabalah, everything beyond the World of Atziluth is considered "unknowable," essentially "negative reality." There are "three veils" that branch outward into the unknowable, called Ain, Ain Soph, and Ain Soph Aur which roughly translate to "Nothingness," "Limitlessness," and "Eternal Light." The Nothingness element gets my Xemnas senses tingling, and my mind goes to Xemnas on that big dragon at the end of KH2 chasing toward some boundless horizon beyond the doors of his artificial Kingdom Hearts. And also, the World That Never Was is sort of Quadratum (i.e., "Unreality")-esque. So, uh, idk, make of all that what you will lol