That's a pretty cool post, with some pretty cool ideas, and the only reason I didn't say so earlier is I didn't want to make a post that was basically just "That's cool."

Anyways, I've had some planar ideas.



It's a common mythological trope to have the world made from the body of some primordial being (Tiamat the sea-dragon, Ymir the primeval giant, Cipactli the crocodile monster with knee-maws). The Great Wheel has a bunch of worlds; why not have a bunch of primordial beings?

Once, there was nothing but pure chaos, a mixture of every kind of energy, merging and splitting endlessly. But even chaos will occasionally produce order, if only by accident. Many primordial creatures formed this way and were ripped apart once more by chaos.
Finally, some beings strong enough to survive the chaos came into existence. The first were the four Elemental Primes, who began to distill and absorb their respective elements from the elemental chaos. Their gluttony created disparity between light and dark, presence and absence, life and dark; this disparity was ignited by two new primal beings, the Positive and Negative Primes, who fed off each other to grow even greater.
The Primes were not benevolent creator gods, yet the peace and stability they created through their hunger was a safe haven for primordial creatures spawned chaos they had yet to consume. Some of these fed on the Primes as parasites, some fed on the energies which the Primes failed to consume, some fed on other primordials. Sixteen grew to rival the Primes in size and power, and another surpassed them; these seventeen are known as the False or Late Primes.

Eventually, beings now known as the Titans came together. They were far smaller than the greatest primordial beings, but were blessed with intelligence and empathy, which allowed them to forge bonds and cooperate, two skills beyond any other primordial beings. The Titans began slaughtering the largest and most dangerous primordial creatures, fashioning their bodies into weapons, fortresses, and other artifacts to ensure their survival and victory. The Titans split apart into sixteen tribes on ideological grounds, but remained cordial rivals at worst.
Eventually, the time came to fell the Primes. Every Titanic tribe banded together for the effort, felling one Prime after another by removing their hearts—the six sparks which originally tamed and consumed the chaos. As the Primes began to fall, the greatest False Prime absorbed its fellows to gather strength. The Titans saw this threat for what it was, and prepared.
They transformed the six bodies of the six Primes into six worlds, which would become the Inner Planes; each had teams of Titans from all sixteen tribes forging weapons, soldiers, and more for the inevitable war effort. Meanwhile, the sixteen elders of the sixteen tribes took the hearts from the Primes and forged them into the Heart of War, perhaps the greatest weapon ever designed: A massive sphere filled with cavernous spaces, in turn packed with raw elemental energy and the machinery needed to shape it.
The day for the great battle came, and after much fighting, the Titans defeated the False Prime. It was a terrible battle; many titans died, and the Heart of War was disabled, but the day was won. The False Prime's heart, a grotesque amalgamation of all the hearts consumed to form the False Prime, was removed and used as a burial shroud for the Heart of War, which was placed in the center of the cosmos as a reminder of that great battle.
The sixteen tribes of Titans sought to create a new home for themselves, but squabbled over what it should be. Eventually, one Titan queen proposed a solution. Each of the lesser False Primes formed a sector of the final False Prime; their bodies would be transformed into homes for one tribe each. The center of the False Prime would be used as a neutral meeting-place for the tribes, and the queen would create a city at its center to serve as the hub of the multiverse. This plan was embraced and put into action.

The Titans forged the Elder Gods in the Inner Planes and set them to work, shaping each of the Outer Planes in turn. When their task was done, they were sent to the mausoleum for the Heart of War, to ensure the Primes true and false never again bothered the multiverse. But this heart of hearts, the heart of the universe, was formed out of chaos given order, out of life and death and the four elements. It would not remain silent forever, and the Elder Gods took advantage of this. They nurtured the growth of life on this universal heart, encouraged the unfathomable energies within to blossom, and prepared to take vengeance on their creators and masters.
The Elder Gods rebelled against the Titans, sealing them away in the hearts of their planes. Only a few survived, by retreating to distant corners of the cosmos or impregnable fortresses. The Elder Gods drove the Titans who fled and nearly all remaining primordial monsters to the edge of the Great Wheel, where the primordial chaos had begun to creep in once more, to the realm now known as the Far Realm.
But the Elder Gods were unable to avoid their creators' folly. The world they created and derived energy from, which they were harnessing to empower the whole multiverse, had new life of its own. Mortals were more numerous and creative than any race to come before; they were therefore able to invent new ways to build and destroy, and the deeds they could accomplish together overshadowed those of any who came before.
Mortals could not hope to fight the Elder Gods alone, but they created their own gods by channeling their mortal energies into ideas and beliefs. The gods of man overthrew the Elder Gods...but the cycle repeated, as the gods demanded that mortals serve them. With all that mortals had poured into the gods, with how their power and creativity and even their ability to cooperate had been tied to them, the mortals had no choice but to accept. And so the world remains.


So...yeah. The Outer and Inner planes are obvious. The Material Plane is obviously the Heart of Hearts, with the old Heart of War being the Underdark and the False-Prime shroud forming the...Overlight, I guess. The Astral Plane remains as the empty space between worlds, littered with primordial artifacts crafted by the Titans.
Not sure how to fit the Transitive Planes into this model. I guess the Ethereal, Feywild, and Shadowfell would stay as reflections of the Material Plane, maybe made after some weird machinery malfunction in the depths of the Heart of War. This might justify there only being one Underdark; the Underwild and Underfell are just different corners of one united Heart of War, which just link to different versions of the Material Plane.