Beneath Ashes, among Chaos
Earth, Africa, more than 5000 years ago:
A youth was seated atop the not all that old golden tipped landing pad. The god-king´s servants supervising the flesh clad souls on each side of the river beneath him paid him no mind, for neither cared for the others actions. His chin resting on the folded fingers of his hands his bored gaze taking in the world before him. Eastward his sight is hooked by a mountain so large it did not fit the curve of the land, sometimes two at a time, its top glimmering like freshly fallen snow, the sun´s always angry glare breaking on the myriad roofed palaces on top of the clouds. The damages, torn into them by the golden King of Apes who rebelled against the world above after the flood, already healing.
He knows, even further east he would see the Eastern Ancient Serpents water besieged yet dry mirror-like crystal cave beneath the height of the waves. The god-king there, wondering when he had left. Outside the borders of physical reasoning he saw the island born from the dust being swept away by the Heavenly Demon´s storms, fertile and peaceful unlike its parent lands yet already poisoned by each other like all products of Ape hands. Growing out of the island and to the north all was shadowed, a trunk so wide eyes could not see where its border. Branches reaching outwards to the easts, wests, norths, and souths, all the way to the sun and all other celestial attendants, some carrying glimmer just like the mountain top, others nothing at all.
And all was planted by Ape hands.
Behind the tree stands its thought-father. Unseen yet misunderstood by Worms, Apes and Monkeys, soul enriching storm streams ever twisting due to its pulse which seeds souls upon the globe of dirt and picks them up again after expiration date.
The Heavenly Demon sitting in the skies snickering at the Apes thoughtlessness on being hindered from trying to built a Tower by the Worms. A Tower in mirror shape to her own, which is piercing the outer realms above us. It serves as her perch ever since she had brought the soul enriching winds with her, and began trading insults with the Worms around her.
But all was already changing again, chaos having no need for stasis. The Floods, having threatening to swallow all land, released upon the shattering of Atzlan, had already receded when the former Age had ended and the current one had still been lying in wait. New realms had sprung up like wheat, hosts of the unliving had fallen and had been raised, in the end nothing had changed to change.
Still not wanting to meet the sun´s always angry glare, he turned around on his seat and watched the plains of the west. There a fiery mountain chased away the hungry hordes which had crawled out of the dark grounds of self pity hidden in the Void Streams with its always angry glare. This was new, even newer than the always angry glare of the sun.
He thus leaps from his own perch, shrinking space under his strides and convinces the dragon necks to carry him onwards only setting food on the planets curst again when he nothing but ashes are reflected in his eyes. At the foot of the fiery mountain stands no Ape nor one such as himself, still half-clad in his burned away meat suit, dragging the giant corpses remaining of yesterdays battle onto the pyre he fought a burning mountain. All around them he witnessed hungry maws fleeing from the flames light and whenever darkness swallowed one of them a burning corpse was thrown after it, for if possible the burned youth would leave no darkness behind but only blissful shade. It was not to be however, for the Houses had already blown the war trumpets and made up their minds, today would be another battle, as always in this age of ever changing twilight. For the corpses were useful twice, as from the dark they also called out the hungry, who were the equivalent of myriad singular souls in the great House-forges.
...
A futile endeavour truly, nonetheless refreshing, for a victory against the inevitable brings about such feelings, short-lived they may be. There they stand side by side, of different nature yet united by their futile task, starring into death-mountain devoured by flames which were torn from the sun herself and thus earning the whole world her ire against the thief.
"What would be your word o´calling, Child of Man?"
That call word was grating on him, and all of his kin, rude as it was even if true. The other was all too aware of that. Of his displeasure as well, after all while using the tones of spirits no falsehoods nor emotions could be masked. Not with their weak skills and great ego at least.
"Virtue of the Surging-Will´s House, and yours? Then I would finally be able to call you something beside Ape."
"... None actually, I never got around getting myself a new name upon reaching age, and my child name is nothing I would like to be called by."
"Khahahahahahahaha! Indeed! Such is the bane of all parent´s children! I understand you are from one of those Ape-tribes which change their name after deeds of great valor?"
"No, I don´t know of any such tribes, however your assumption of our custom is correct."
"Then how about I give you a new calling? For I do not wish to call one such as yourself neither as boy nor Ape."
"Do as you will. But be prepared to make your bed atop the fallen should this turn out to be another one of your so not funny jokes."
"I´ll keep that in mind, Monkey. Here we stand in the shadow and ashes of the corpse mountain you piled up, ichor and flames decorate it like the volcanoes of your home planet, and I need not seal my eyes to spew out a prophecy how it will not be your last. Thus you shall be called Mountain-Maker from Origin of Mountains (Yamamoto). Happy? Or shall I start hiding behind the pillars of my House while quivering?"
"Passable."
"Khahaha! Now then that we decided not to kill each other over the calling of a single being, fare well and let us convene again on a field of crimson blossoms and thorns such as this, Mountain-Maker."
This should represent the spiritual chaos across Earth before the Soul Society was formed. It creates a reason why Earth would have more spiritual activity than other planets, as some kind of alien, the Heavenly Demon migrated to Earth and the fallout of his spirit enriched the souls of all those living on Earth. Like Ichigo to his friends, just planet wide.
Furthermore its purpose was to enable me to write a nearly unlimited number of crossovers by creating a coherent world system where this is possible as I do not want to resort to the "multiverse solution", thus there are references to aliens, mythological pantheons, or other original fictions: TES, Fate Stay Night, Stargate, Feng Shen Ji. This was achieved by giving the living beings the power to make mythopeia reality, aka realise the mythology they believe in. Gods born from belief. The comments of Yhwach regarding the Soul King make me believe that the SK was somehow created by precursors or very old Shinigami to quell a sort of chaos. Yamamoto as the oldest Shinigami known to us is already in here, even if 5000 years is probably far longer than he is in canon considering that 2000 years ago he was already past his prime it is not even that unlikely. My thought was that the creators of the Soul Society somehow collected all that spiritual fallout across the planet and then hit the Soul King into that newly made realm like a nail to keep it fixed. The Dangai was then created as a bridge but also as a means to keep Earth and the Soul Society at a fixed distance.
As for the World Tree, which probably everyone noticed, and its father: the world tree is something that turns up in fiction and myth quite often, but I wanted something more metaphysical and universal for this. Thus I used human belief to misinterpret the "father" into a World Tree. The "father" in turn is the reimyaku/ley lines/dragon veins etc and also the circle of reincarnation itself. Think of it as a large stream coming out of the crust of the Earth, or the Jureichi?, and then separating into countless branches creating the image of a giant tree. Yamata no Orochi, dragons (in Asia often river/water spirits) also are encompassed within this system: Tree = many reimyaku etc as river of souls, each river a dragon.
And last but not least: golden Gilgamesh as King of Humanity who rebelled against Heaven = golden Son Goku as Monkey King (for what are Humans to gods but monkeys) who rebelled against Heaven
With this cosmological set up any kind of crossover should be possible without resorting to needing to let the characters cross dimensions/realities etc. The need for a multiverse which for some reason has hardly any differences should thus be eliminated.