But really, someone needs to make an actual story based on this premise.
Oneshot: Sauron of Chaos
The day was over, the battle was lost. A foul Taint had struck into the hearts of the pure. A dark Shadow had fallen over the lands. The brave Men, the proud Elves, and even the stout Dwarfs, all had been scattered and swept aside. The Last Alliance had been vanquished, and the free people that remained were forced into hiding, becoming little more than raiders and scavengers - or slaves. The wise and powerful Istari were destroyed from the inside, by corruption and treachery. The Shire, once a bastion of nature and happiness, of greens and plenty, became a fortress of cold metal, of infinite greys and endless drums, of infernal machinery fuelling the eternal war effort.
The Light of the Valar was replaced by the unholy red, purple, and black of grim evil. The night was perpetual, and what little light that managed to penetrate the black veil from Mordor had a wholly demonic quality to it. Reds and purples, it drove away hope in favour of doom, desperation, and insanity. And from Mordor, from deep inside the tall black fortress of Barad-dûr, Sauron ruled all with an iron fist. An iron fist, with a curious golden artefact on one of his fingers; a ring.
Now, while a few tribes remain - mostly Rangers, joined by the occasional Elf - most of the populace of Middle-Earth has been crushed. Utterly and completely crushed, bereft of spirit or hope, bound to eternal slavery. Literally; the Necromancers of Mirkwood, operating from Dol Guldur, would make sure of that.
The Dwarfs had long since died out, their bones rotting away in their underground caverns, occasionally gnashed to dust by Goblins. The Elves were among the most resistant to the foul energies from Mordor, and recognised the danger with clarity of sight and purpose. Those that could remember the tales of old, speaking of ancient wisdom-turned-madness and floating Craftworlds, desperately tried to seek them out, to warn them of the coming evil. Most of the Elves, however, drift lifelessly throughout space, their fragile bodies being torn apart by gravitational forces. The Men have mostly been enslaved or gone mad - or usually, both - but some few roam the lands, surviving in independence. A brutal and savage existence, they have become the very monsters they still attempt to fight. The Hobbits were almost entirely immune, but a life of literal nightmares at the hand of Morgul-blades keep them at bay, tortured slaves to fuel the forges of Sauron.
And so it was that the Third Age came to a close. The Fourth and Final Age began the moment Sauron's grasp exceeded the confines of Middle-Earth, the cradle of Arda, into space itself. But space was not a new frontier for Sauron, Greater Daemon, most loyal and most powerful servant of Khorne.
"Speak, Inquisitor!" Boomed the all-terrifying voice of the Greater Daemon Sauron, literally tearing apart lesser souls.
"My lord," To his credit, Saruman didn't shiver nor cower. "The Nazgûl are ready."
And on the horizon, colossal black bodies could be seen, rising, ascending from Middle-Earth into the boundless voids of space. Black bodies, of metal, crackling with energy, filled to the brim with Orc, Goblin, Troll, Mûmakil, Ungoliant, Giant, Balrog, and a whole arsenal of other foul creatures. Not to mention the human contribution; the explosives of the Order of the White Hand, the magic of the Black Númenóreans, the mad but devout swarm of Cultists and other worshippers... All were set to leave, to set foot on the path that would lead to the subjugation of all life.
"Khorne desires blood!" Sauron roared, malicious laughter echoing deep into the depths of the Warp. Yes, with the mad energies of Chaos awaking Tomb World after Tomb World, with more and more soulless and bloodless bodies joining the onslaught of Chaos, none could oppose Khorne and His minions... So long as these minions remained loyal to Khorne, that is.
All would fall. Blood for the Blood God. Skulls for the Skull Throne. And One Ring to rule them all, in the end. All would fall...