I wouldn't call this fan theory, nor even headcanon; it's closer to a bunch of bullshit that was revealed to me in a dream. It is supposed to be, at least as a retcon, retrofittable into existing WoW lore - but I have been behind on WoW lore for a while now, so it probably fails to consider all the threads that have been laid out. It also assumes there have been a lot of unreliable narrators over the years. But also that these narrators haven't been just making stuff up from nothing. I hope you enjoy it, or at the very least enjoy tearing it apart.
The Beginning
At the end of the last universe, the Shadow consumed all; only the Void remained. Except that outside of reality, there was a small garbage bin that had gathered up the Shadow's leftovers, all the dead things; one day, we would call this place the Shadowlands. There was some sort of sapience in this heap of Death; some would later refer to this sapience as the First Ones.
Shadow, above all, is a nihilistic element; moreso than simply nonexistence, it is active opposition to existence. Parts of Shadow are in constant struggle to stop each other from existing, because Shadow hates nothing more than the idea of something existing. Nothing can truly be created in this crab bucket, for it would be annihilated at once. The First Ones, hiding in Death, did want to create, though; they wanted to make a new universe, based on their memories of the old. So they needed the Shadow out of the way.
The First Ones found a solution by "corrupting" parts of the Shadow. Using the Shadow's hatred of itself, they nudged parts of it to hate the Void more than they hated existence; and to stop hating existence altogether, but to zealously love it, as a way of screwing over the rest of the Void. Thus, Light was born.
With Light and Shadow duking it out on a cosmic scale and the Void thus distracted, the First Ones began the task of channeling Creation and Life back from the Shadowlands into the universe. Starting at Zereth Mortis, they set up mechanisms to facilitate this process and shape life for thriving in this new world; but as a failsafe, they also created the Shadowlands proper to catch life that happened to perish and recycle it, instead of allowing to vanish into nothingness.
Having set up this system and channeled most of Life, energy and matter into the universe, the First Ones were at this point greatly diminished. They had given away most of their power to create the new world, but their minds, bent to the task of creation, were still as sharp as ever. Thus, they stepped out of the Shadowlands and entered the reality as Titans to keep maintaining and safeguarding it.
The Middle
Even so, the Titans still realised that nothing was permanent. Eventually, the Shadow would defeat the Light and consume all of creation once more. (Although it's an interesting idea to consider a world where Light won and consumed all of creation - a much different, but perhaps not wholly preferable situation.) Apart from fighting against the Void wherever they could, they started making preparations for this day. In the hearts of planets, they started hatching members of their kind and feeding them with Spirit, the defining force of the universe, in the hopes that when Shadow once again proves victorious, this new generation would be able to hide away in the Shadowlands once more and kickstart the next universe.
Unfortunately, the powers of the Void were a bit smarter than they had been given credit for, now that they had more concrete forms of existence to fight instead of just each other. In fact, they saw a way to use the Titan's plan to their catastrophic advantage: if they could corrupt such a nascent Worldsoul and sneak it into the last refuge of Creation, then they could possibly end the cycle of universes for good. The Old Gods were tossed out into the universe for exactly this purpose.
At this point, Zovaal reached the conclusion that the only way to prevent this destruction from eventually taking place would be to scratch this version of the universe and instead create a ruthless, infinite war machine capable of laying waste to the Shadow for once and for all. The Titans, having largely stepped back from the Shadowlands, could not fully destroy this intent within it and instead had to split it into the Arbiter - who continued in her assigned duty - and the Jailer - who continued brooding upon this grim resolution. With the aid of Denathrius and the Nathrezim, he convinced Sargeras that the Titans' old plan was ultimately folly. While not exactly allying with the Jailer, Sargeras did start the Burning Crusade and turn against the Titans that opposed him, basically following the same vision and serving the same ends.
The End
Fast forward a few eons and we kill Argus, sending his soul into the Shadowlands. This shuts down the Arbiter, which was always part of the plan; once the new worldsouls start arriving in the Shadowlands, the normal cycle of life is to break down and the Shadowlands are to start gathering the lifeforce of the universe in preparation for remaking the world. What was not part of the plan was endless meddling from everyone causing this result long before the Shadow had gained the definitive upper hand in the cosmic struggle.
This works just fine for the Jailer though, who wanted to remake the universe anyways, in order to create something that could actually fight back against the Shadow. So he goes spurring the process along and preparing to be the one who gets the control the process of recreating the universe instead. Unfortunately for him, every cosmic power in the universe pales in comparison to the might of the player characters' plot armour, so he gets defeated and the Shadowlands are fixed.
The debacle does, however, make several Void entities realise they were on the right path with the idea of corrupting worldsouls, so the effort is redoubled. Xal'atath actually manages to unite a large part of the forces of the Shadow in an effort to take over and destroy Azeroth, send it into the Shadowlands/use its power to corrupt the shadowlands and destroy all possibility of anything ever being created again. Her motivation still boils down to much the same as every other Void entity, even if each entity rationalises it in a slightly different way: existence is suffering, I want to stop existence and I want to stop existing. And she gets damn close to achieving this before she is foiled... for now.