Hypothesis:
1.The Great Dark Beyond is born as a result of the cosmic conflict between Light and Shadow. In this moment, masses of energy are flung out into the physical universe. Worldsouls. These are inherently of the physical universe, and as such belong to no cosmic force in particular.
They are of reality.
The worldsouls are attracted to the warmth of other nearby energy sources: stars. Like nebulae, the energy of the worldsoul is almost like a nursery.
Over time, solid matter begins to coalesce around the worldsoul, and a planet comes to be. Azeroth.
2. One of the shards of Light sent out in the dawn of the cosmos finally hits Azeroth.
Life is born, and first among it are the Elementals, and myriad of other creatures. Perhaps even the ancestors of the Tauren and Trolls?
The shard's energies eventually turn Azeroth bright and luminous.
Interestingly there does seem to be a close relationship between the forces of Light and Life, as evidenced by Chronicles Vol. 1, which says the Naaru are always on the lookout to protect Life. So there's some form of symbiosis going on.
Over time, as the worlssoul grows, it consumes Spirit. The Elementals become increasingly unruly and wage wars on each other.
3. A number of Old Gods also finally collide with Azeroth, 5 to be precise. They begin burying themselves in the planet, their long tendrils reaching far across its surface and deep below. The worldsoul is almost overtaken by Void. The Elementals, and many other living beings, are enslaved. The Black Empire is formed. However, infighting is common and the prophecy says only one of them will remain to consume the worldsoul.
One of them, called Xal'atath, is slain by her brethren.
4. Aggramar discovers Azeroth, and eventually the Pantheon of Titans arrive, realising the peril that the worldsoul currently finds itself in. But also the opportunity it presents.
They construct the Manifold, shape oceans and mountains to contain the Old Gods as best they can. But they also imprison them, as well as the worlssoul, giving their machines time to work. The worldsoul takes on a blue shimmer, affected by Order magic.
5. With the Old Gods defeated, the Light through Elune reaches out to Eonar and reveals to her the truth of the cosmos. They form an alliance and find affection for one another.
Together, they device to steer the sleeping worldsoul's development in as good a direction as is possible.
So they tap into the realms of Life and Death, shaping the Emerald Dream and parts of the Shadowlands to their purpose. Wild Gods will roam Azeroth to protect it, and great world trees will bury their roots deep into the soil, calming the worldsoul and protecting it from the imprisoned Old Gods' influence.
From the Well of Eternity, they draw from Azeroth's own essence to create and evolve life. The Kaldorei are born, and are meant to aid in the defense of Azeroth as wardens of the world trees, and watchers of the Old Gods' minions.
Outraged, Aman'Thul tears down Elun'Ahir, fearing it would destabilise their titan-forged creations and disrupt the construction of the Manifold. He sees many futures and knows of the impact rampant Life might have.
6. Fast-forward through the last ten thousand years or so, and the forces of Shadow, Disorder and Death will many times over seek to destabilise the current world Order, in defiance of Order, Light and Life which currently hold the most sway on the worldsoul.
The Burning Crusade fails.
The Scourge fails.
The Hour of Twilight fails... or does it?
As Xal'atath once said, defeat often benefits N'Zoth.
With the Titans' world order coming crumbling down in the Worldsoul Saga, the roots of Elun'Ahir corrupted by Old God blood, and the Light giving way to Nightfall... Midnight awaits!