This'll be the last part, unfortunately. I don't want to hog all the space on this thread! XD
This last part will be the summary of the (unfinished) plot that I have in mind, including major events and all. While doing that, I might as well explain how the Farplane would look like if it were seen in a game.
The idea of a design I have for the Farplane is basically to just make it look very, very colourful and weird. Different sections of the Farplane will have different appearances and whatnot. (Like, in FFX when you enter the Farplane, the sky is yellow with tons of flowers and waterfalls everywhere, and also has a blue sun for some reason. In X-2 when you actually get to explore it, it mainly consists of floating islands and purple flowers, and the sky's very dark and has the Aurora Borealis in it. Maybe deep within the Farplane, the land breaks up and it starts getting more dangerous, or maybe it's to do with the Farplane instability at the time of X-2.) In terms of layout, the Farplane is divided into 'sectors' which defines the different levels of altitude, and, like Spira, the land is divided into two parts: the Djose Continent and the Wilderia Continent. The closer you get to Djose, the more towns there are, whereas Wilderia has lots of open space and little to no towns. In that way, the Farplane is kind of like an inverted alternate Spira, only obviously with different landmarks and continent shapes.
So it'd be like:
Troposector (Low): This is where the mainland lies, along with a few low-hanging floating islands and some small mountains.
Stratosector (Medium): The altitude where most of the floating islands hang within, as well as some flying carrier vehicles and birds. The rest of the taller mountains end here, with the exception of one. This sector is also where Equinox is.
Mesosector (High): Very few floating islands reside here, but are said to contain ancient treasures within them. This is largely known as the 'Angel Habitat' where any person who comes within range is said to have some of their pyreflies condense into wings, in which they can fly around in part of the Mesosector with. The tallest mountain in the Farplane, the Aero Mesa, ends here. People climb up to the top of the Aero Mesa so that they can sprout wings and experience the Angel Habitat.
Thermosector (Very High): Nothing is said to reside in the area above the clouds. The air is so thin, it's nearly impossible to survive. It's rumoured that this sector once acted as a portal to the Fiend World, but since the connection's been severed, nobody knows what is up there now.
Now, to conclude the story with a summary:
Tidus crashes onto the mainland, and Shuyin and Cirrus are nowhere to be found. He's in a field, and his crash landing attracts quite a few tourists. They make way for two more main characters: Auron and Braska, whom were on the way to a mountain town for a meeting. They get away from the other people and get acquainted again. They ask where Jecht is, Tidus says that it's a long story, and that they should head to a town before doing anything. He looks up to where he was falling from, and finds out that Equinox is being covered by the clouds. It must have been a long drop.
So they get to the town, Tidus tells them everything, and they conclude by saying that they need to rescue Jecht. Before they head out into the public, they say that Tidus needs to change out of his clothes. (they're ripped and dirty because of what happened) He then recalls a package that Valefor gave him, and he opens it, revealing a strange blue shapeshifting cloth. The note on it says that this was the best material that Shiva could make, and that he needs to look at least decently royal in public, seeing as how he defeated Sin and all. Cue Tidus wearing it like a trenchcoat. Now we have his permanent look, and the game finally explains the Tier system.
We get introduced to shops and interactions, Tidus gets a proper sword made out of the metal that he was struck with, yadda yadda, let's move on. The group finds Cirrus and Shuyin in a nearby jungle, and at first Shuyin doesn't recognise Tidus. He's noticeably more shaken and paranoid about his surroundings, and seems jumpy at every little thing. Some fiend attacks later, Shuyin joins the party permanently, though it's just to be kept safe from the fiends. Now it's on to the next town.
Some events later, the town gets invaded by the strange fiends that attacked Equinox. It's then we're introduced to Liquid Nightmare fiends, though dubbed 'Mysterious fiends' at this time. Think of the oversouled fiends that you find during X-2. Now imagine them stronger, and not possible to be defeated at this point. So the main objective is to just avoid the fiends and find a safe spot. Tidus has short flashbacks to when he was escaping from Dream Zanarkand, and they then bump into a strange human-like but monstrous looking person, introducing himself as a Nightmare Core. He then takes off, claiming to have fulfilled his purpose. It's then they realise that almost all the townsfolk have been annihilated, and were being carried into a rift by the other Liquid Nightmare fiends.
Since I don't know what to do after this, I'll skip to the major story parts. They run into the machine-like monster again, and funnily enough, Jecht managed to hold onto its grasp all this time. They rescue him. They all reunite, and Jecht joins the party. They then remember the party they must go to, so that happens. At the party, after some cutscenes, Shuyin runs into Lenne, his old partner and love interest. They also find Aeshma at the party, and he talks to them in private about Liquid Nightmare, and how he witnessed its rampage thousands of years ago. He explains Nightmare Cores, and theorises that they're raiding hateful towns and capturing dead spirits to turn them into Liquid Nightmare fiends for their massive army. Shuyin remembers Equinox, and asks why they would raid his home. Aeshma doesn't know, since he's just theorising, but then suggests that they try to go to Equinox and defeat whatever Nightmare Core resides there. He directs them to go to the top of Aero Mesa and drop down to Equinox from the Angel Habitat. but not before enhancing their weapons with 'purity energy' to counteract Liquid Nightmare and defeat the fiends. Tidus then remembers the gas that Shuyin had been hit with that time, and reminds him about it. He replies by saying he never saw that kind of gas before, but Tidus begins to wonder.
Since Shuyin and Lenne are pyremancers, they manage to, after convincing Tidus and Jecht to give up some of their plasma, (the unknown pathogen seems to be a magic amplifier carried over from the fayth) enhance the party's weapons with purity energy. Everybody's weapons now gets a special multiplier against Liquid Nightmare enemies, as well as the ability to actually defeat them. They get to Equinox, kill the Nightmare Core, another vague tragic-villain dialogue, Equinox is saved. By this point, Shuyin becomes braver and more attached with the party, deciding to stay with them, but keep Cirrus behind to keep her safe. Lenne stays behind, too, glad to be home after all this time.
Meanwhile throughout the whole journey, Tidus is having weird dream sequences about a spirit talking to him. Near the end of the game, we get a clear picture of what he looks like. He also looks very similar to Tidus, but somewhat closer to Shuyin. When Tidus gets knocked out at one point by a now-insane Shuyin, he enters another dream sequence. Yet, it feels real somehow. The spirit reveals himself as a Miracle, a entirely pure soul free of Nightmare energy, and was also part of an experiment in Zanarkand long ago.
About 3000 years ago, when machina had advanced and was officially given its own term, an elitist group of scientists lead by the leader of Bevelle at the time was working on the invincible human being: one that would never turn into a fiend and could be resurrected from the dead easily. A not-dead Shuyin at this time was a pyremancer-in-training, and upon stumbling onto the secret project, volunteered in hopes to learn more about the arts of pyremancery. They led him to the gigantic machina contraption, which was designed to break down pyreflies into particles. The team planned to extract the purity energy, amplify it with a pathogen found in the aeons, and give the energy back to Shuyin. But then something went wrong. They managed to extract the miracle spirit, but the Nightmare in Shuyin immediately took advantage of the empty space and he went wild. He trashed the place, killed the scientists, and ran away to the Calm Lands, where the energy proved too much for him and he died. Rhiak found his body, and the Liquid Nightmare in Shuyin naturally made a pact with him. With the Purity and the Nightmare energy gone, Shuyin's body was now an empty vessel, with only his memories before the accident to accompany him to the Farplane.
When the fayth created Dream Zanarkand, the Miracle was drawn to Tidus since he coincidentally looked just like Shuyin. He rested within his spirit, only awakening when he sensed Shuyin nearby. Tidus takes the chance to ask one nagging question: why did he look just like Shuyin? The Miracle suggests that maybe Tidus, being a dream based off multiple people in Zanarkand, just managed to look similar to him. Or maybe the Miracle influenced his appearance over the years since they had become one person in some way. He says that there might be a way to save Shuyin from becoming a Liquid Nightmare fiend, and that is to use the Miracle's spirit and let it merge with Shuyin's body so that he can become a balanced spirit once again.
They go to Lenne and upon hearing the predicament, agrees right away to extract the Miracle. They face off against Vegnagun (the machina used in the project turned into a life-draining superweapon that's been affecting the Farplane) and follow it through the rift into the Farplane Core, turned into a facility that makes Liquid Nightmare fiends. Auron, Jecht and Braska face off against Rhiak at the heart (he conjoined with Vegnagun, making a Nightmare Core/machina hybrid) while Tidus goes to find Shuyin, but it's too late. He couldn't hold out and became a Liquid Nightmare. Meanwhile Rhiak reveals his plan.
He hid in the Farplane Core all these years, after using a newly-built Vegnagun from a thousand years ago to blast a hole in Bevelle's Underground and escaping to the Core. He then made his way around, finding malicious souls to build his secret army. With the help of Sin killing everybody, and everybody's fear and hatred of it, they built a Liquid Nightmare clan in no time, and waited until Sin was dead and everybody was distracted to unleash their chaos. Rhiak wanted to quench his thirst for bloodshed, as developed from his childhood. He wanted to rip open the portal to the Fiend World again and merge two realms of the dead into one, creating more bloodshed between the two species. He would then make the entire Farplane his army, and send it to invade Spira, making the greatest war humanity would ever see. It would be an apocalyptic world full of insanity, and when another species rebuilt Spira up, he would do it all again. And again. Until he died.
The Liquid Nightmare in the facility would then merge with Vegnagun, ejecting all four of the warriors out and flying away up into the sky. And then something about the aeons/fayth coming and the Miracle merging with Tidus to make Purity Aeons, then they set off to fight the god-like machina and Rhiak in an epic final battle.
The facility is destroyed with a gigantic pure attack, restoring the Farplane Core to its original beauty as the Liquid Nightmare fiends are turned back to normal. The aeons celebrate, and the others find the destroyed Vegnagun along with a shaken, broken Rhiak turned back to normal. Meanwhile, Tidus lands on the top chamber of Vegnagun, beaten, ripped and tired from the battle. His sword is also broken and unusable. He hears Shuyin on the other side. Miracle comes out, greatly used up from the last fight, and tells Tidus that Shuyin hasn't turned back to normal; the Liquid Nightmare became his whole spirit. He needs to merge with Shuyin. However, Miracle tells Tidus something: if the Purity energy invades his spirit, it has a great chance of wiping out Shuyin's acquired memories after he got hit by the gas, which means he wouldn't remember anything about the adventure, apart from meeting Tidus. He grimly accepts this, them having developing a brotherly bond throughout the journey. Miracle then says that it's been an honour getting to know him and his life, and disappears for the final time.
He confronts Shuyin, they have a last quick talk, and he turns into a large Liquid Nightmare fiend. Realising that he doesn't have a weapon, Tidus has to fight with his fists. Cue epic final battle that starts you off as weak, yet you get stronger as you fight more. Tidus then manages to rip open Shuyin's skin, summon Miracle's spirit and blast it right into his heart. He turns back to normal, his chest still open, they have a last nonsensical talk as his memories fade, and they both pass out.
Cue the ending with Shuyin in a hospital bed, him being innocent and not remembering anything, everybody walks in, Shuyin just barely remembers Tidus and they reconcile, some other things, ending speech, yadda yadda, credits. Post-ending scene with a first-person view emerging from the ocean while looking straight up into the sky, 'The End' scene.
If this was a game, I'd probably name it after my fanfiction: "Final Fantasy X: Road of Avidity." As in, the road to getting the power to beat up Liquid Nightmare. Yeah, it's pretty sucky. XD
Holy crud, did it really take an hour and a half to write this?