I saw a pitch for this idea from RexBlazer1 in the forums. However, I realized that I misunderstood the challenge and switched up who was supposed to be playing who. ^^; So this is what I initially wrote, and I will put RexBlazer1's challenge up as well.

Oh, also, the average life spans of the characters are closer to those in MK, just so some characters could be rolled into one. You'll see what I mean.

Water, earth, fire, and air.

Long ago, the four realms used to live in harmony. Edenia, home of the vast expanses of beautiful glaciers and tundra, represented water. Orderrealm, distinguished by being an incredibly advanced civilization, although governed by an iron fist, identified with earth. Outworld, known for its barren terrain and awe-inspiring volcanic formations, was connected heavily with fire. And Earthrealm, a beautiful landscape rich in biodiversity, which was inhabited by scattered sects of peaceful monks, who identified closely with air. These realms, though separate, lived together in harmony.

Then everything changed when Outworld attacked.

Having acquired the throne, Outworld emperor Shao Kahn took it upon himself to begin a long campaign of conquest over the other realms, declaring his own homeland greater than the other, inferior realms. But he couldn't do it, having been held back by the rules of a centuries-old ritual known interdimensionally as Mortal Kombat, a competition started by a counsel of ageless beings known as the Elder Gods in order to maintain peace between the realms. To rightfully invade another realm, the realm that wished to conquer another was required to win ten consecutive Mortal Kombats. To enforce these rules, there existed only one being who could manipulate all four elements: the Avatar, who had successfully managed to maintain peace throughout history.

Kahn, knowing that the Avatar remained the only true threat to his plans, grudgingly agreed to participate in the tournament. Much to the dismay of the inhabitants of the other realms, however, Kahn's forces managed to claw their ways to the winner's circle every year, centennially bringing Kahn's dreams of invasion closer to fruition. Then, mere months after Kahn's ninth victory, the then-current avatar, Raiden, died in a volcanic explosion. Despite originally being from Outworld himself, Raiden had vehemently opposed Kahn's plans for domination, his status as the Avatar forbidding him from aiding one realm over another.

Kahn, taking the initiative after Raiden's death, forsook the diplomatic approach and settled for direct invasion of the other realms. Knowing that Raiden had already ascended to the pantheon of the Elder Gods like the Avatars before him, Kahn set his sights on Earthrealm, which would be the home of the next Avatar according to the elemental cycle. Like Raiden, Kahn's queen, Sindel, had also opposed his plans, but she vanished under mysterious circumstances shortly thereafter, never to be heard from again. Using the power bestowed by Onaga's Comet, which came around every one thousand years, Shao Kahn successfully eradicated the entirety of the Air Nomads, eliminating his only obstacle. Or so he thought.

Noticing immediately that another Avatar had not been reborn in Edenia, Shao Kahn instantly knew that he had missed his target. The new Avatar was out there, somewhere, hiding. He made it his life goal to find the one that had gotten away, but after centuries of having no luck accomplishing this goal, he sent his only son, known only as Prince Scorpion, out on the mission instead, accompanied by his bumbling failure of a war general of an uncle, Bo "Rai" Cho. The prince had the same luck as his father in finding his prey, but persisted in the name of regaining his honor and being able to return home someday.

It would be millennia later before a team of two brothers from Edenia would solve the mystery of what had happened to the Avatar. Kuai Liang and Bi Han, known in their tribe as the Sub-Zero brothers, stumbled upon an abnormal glacier while hunting one day. They quickly noticed someone was trapped inside, and using their combined might, managed to break the glacier open, releasing the captive. Much to their surprise, it was only a young boy, even more shockingly dressed in the distinctive garb of the extinct Air Nomads. Realizing that they had found the Avatar, who introduced himself as a simple monk named Kung Lao, the two brothers related to him the horrors of what had occurred in the last five thousand years. Kung Lao was devastated to learn of the destruction Shao Kahn's reign of terror had caused; he had already managed to successfully invade and establish colonies in the other realms. Perhaps the most crushing blow came in the form of the news of the death of Kung Lao's people, the Air Nomads. Disheartened, but now realizing his duty to the realms, Kung Lao and the Sub-Zero brothers agreed to circumnavigate the globe as a team in order for Kung Lao to receive proper training in the art of bending the other three elements.

Unfortunately, they were not alone for most of their quest. Word spread through the realms quickly about the return of the long-lost Avatar, who came to be known colloquially as "the Great Kung Lao." Upon hearing the news that the Avatar had returned, Prince Scorpion remained hot on their trail for most of their journey, finding ways to catch up with them wherever they went. He trailed them from Edenia, where Kung Lao received his waterbending training (before Outworld entered into the equation and conquered most of the realm), all the way to Orderrealm, where the trio found a new recruit to teach the Avatar earthbending: a young blind girl named Jade (Kung Lao also received spiritual advice and training from his past lives periodically, most prominently Raiden). To make matters worse, Shao Kahn, evidently tired of his "idiot" son's failed attempts to capture the Avatar, sent his more competent daughter, Mileena, out to finish his work for him. Mileena, together with her Tarkatan hordes and desperate for her father's approval, proved an effective and deadly foe to her targets, both the Avatar's gang and her own brother.

Matters came to a head in Lei Chen, the capital city of Orderrealm. Although it boasted a wall that had managed successfully to kept Shao Kahn's forces out for the duration of the war, the seemingly safe Lei Chen concealed within its great walls a dark secret; a conspiracy headed by the cultural police that kept its residents ignorant of the war outside. The Great Kung Lao and his followers, originally planning to ally themselves with the Orderrealm emperor, unearthed the conspiracy headed by Shang Tsung. To make matters worse, Princess Mileena had managed to catch up with the Avatar and his gang, and quickly put Shang Tsung out of power as the head of government in Lei Chen. Prince Scorpion, having caught up with everyone else after being on the run from his own homeland for so long, abandoned his uncle Bo Rai Cho and accepted his sister's offer to return home with his honor. Having only narrowly managing to escape, the Avatar gang fled from Lei Chen, the city having been overthrown in the name of Outworld and their hopes for a large scale invasion dashed. They resolved to perform a smaller-scale invasion themselves on the Day of Black Sun, with the help of allies they had accrued along the way, but their attempts were rendered in vain as Shao Kahn's forces proved to be too much for them. They retreated to Earthrealm, to the site of the temples that had formerly held the Great Kung Lao's people. Although the situation seemed bleak, there was a silver lining; Prince Scorpion realized the error of his ways and defected from his father's side, opting to join the Avatar's crew in the fight against Outworld. Although many in the group, especially the Sub-Zero brothers, initially had their reservations about letting their former enemy in, Scorpion's change of heart proved to be genuine, and he was able to fill the role as the Great Kung Lao's firebending mentor.

Finally, the day came upon which the fate of all of the realms rested. Onaga's Comet returned once more, giving firebenders everywhere an immense boost of power. Shao Kahn planned to use this power to once again invade another realm, this time setting his sights on all of Orderrealm. This time, however, the Avatar's forces were ready. Everyone picked their fight; Bi Han and Jade worked to eliminate Shao Kahn's air forces, while back in Outworld Kuai Liang helped Scorpion faced off for the final time against his sister, who had been rewarded with her father's former position as Fire Lord. However, Mileena had grown increasingly paranoid and delusional within the time she had been assigned her position, and was being pushed further to the brink of insanity by a combination of acknowledging her status as a figure head, betrayal by her Tarkatan forces (especially from their leader, Baraka), and her jealousy of Scorpion's status as their mother's "favorite." Her defeat at the hands of Scorpion pushed Mileena to her breaking point, causing her to lose her sanity as Scorpion re-usurped the throne. In addition, the crew received significant assistance from the White Lotus Society, a secret group founded by Raiden thousands of years ago consisting of members from all four realms who wished to reestablish order, led by Bo Rai Cho as they worked to take back Lei Chen.

While all of this occurred simultaneously, the most important battle reached its peak. The Great Kung Lao, although initially not being able to withstand Shao Kahn's assault, was quickly pushed to the brink and unlocked control of the Avatar State. His power exponentially increased far beyond that of Shao Kahn, the former defeated the latter and was able to remove Shao Kahn's bending as a result of his newfound abilities.

Their threat eliminated, the realms were once again at peace. Scorpion reunited with Bo Rai Cho, the latter forgiving the former, who realized that his uncle had acted as more of a real father to him his whole life. Scorpion discovered the truth about what happened to Sindel, and the Sub Zero brothers returned to Edenia with the intention of reestablishing connection with the other realms. The Great Kung Lao, now a fully realized Avatar, reinstated Mortal Kombat, thereby guaranteeing the safety of the realms for years to come.

Together with Fire Lord Scorpion, the Avatar and his followers established Republic City, a town in Outworld that attracted the most brilliant of the realms' engineering minds. Within less than a century the village experienced a massive spike in industrialization and manufacturing, making it the largest and busiest city in all of the realms, surpassing even Lei Chen. The Great Kung Lao married an Outworld priestess and had two sons, Liu Kang and Kung Lao, who went on to become star athletes in Republic City's main sporting attraction, Pro Bending. Shortly after their births, however, the Great Kung Lao passed away, leaving his legacy in the form of the promising new Republic City, which continues to grow with each passing day.

Shortly after the Great Kung Lao's passing, a new Avatar was born in Edenia. Only six years passed before a young girl from a powerful tribe, called Kitana, discovered her powers and was visited by the high priests of the White Lotus. Having mastered her bending abilities exceptionally early, the young Avatar has only to go to Republic City to meet her airbending teacher, Liu Kang. Although her mentor is not much older than she is, it will take immense dedication for Kitana to become a fully realized Avatar like the Great Kung Lao before her.

However, a new threat is brewing in Republic City. An underground society called the Equalists, led by a mysterious man known only as Havik, is gaining momentum, along with their radical ideas about eliminating benders altogether. Councilman Hotaru's plans for dealing with the Equalists reflect a skewed sense of morality not much better than Havik's, and it will take the help of Chief of Police Sonya Blade and Kitana's two new allies, Liu Kang and Kung Lao, to stop the new threat to order between the realms.

I'm actually pretty surprised with how well the two series fit together (both even have a White Lotus Society; fun fact: that actually existed :D). Obviously I skimmed over some major events for the sake of time and clarity, and some character relationships had to be changed, but I don't think I mucked it up too badly.

Originally, I was planning to have Kitana fill Scorpion's place, just to fit in with the whole "child of royalty has been duped all their life and learns to see the light" thing, especially since Mileena was the sister, but I opted for Scorpion instead because of the fire thing (and also because Kitana fits into way too many roles in my view, such as those of Zuko, Korra, and Katara, which is why I changed the last role to one of the Sub-Zero brothers). I also kept ages the way they were in Mortal Kombat so Ozai and Sozin could be rolled into Shao Kahn.

I was also originally going to have Johnny and Fujin fit in to the Legend of Korra arc, with Fujin filling Tenzin's role, but there wasn't really a way to squeeze them in there without seeming forced. I kept Sonya with the Korra arc to reflect the more modern setting, since the name "Sonya Blade" seems a little out of place in a world based on imperial China. :P

So, yeah. Thanks again to RexBlazer1 for the story idea, but I think I'm going to leave this here. There's simply no way that I would be able to finish a story like this with all the character relationships and plot twists that both series demand. I wouldn't mind passing the baton on this to anyone who seems interested (maybe they could work in a Johnny/Sonya angle. :P Just throwing it out there). But for now, this is where I check out. Ciao!