In Vitro
This is my take/prediction/whatever on what happens to the Guardians of Light while searching for Sora, and my vision for KH4. However, my story goes outside Disney IP, for personal preference and more expansive storytelling.
Canon compliant where it counts. You'll know where my own tastes and ideas come into play.
Prelude: Faded Glory
You know, what all is said and done, one would assume fading back into the pits of darkness after so long with be a smooth transition. You finally let go all the pain you endured against your will. You are set free of the chains those around you set onto your wrists. You achieve the freedom that comes with the blissful oblivion of your choosing.
"MOTHERFUCKER, WHY WON'T YOU JUST DIE ALREADY?!"
He was so wrong.
He once thought long and hard (of course, however long and hard it was didn't really matter, because to him, there were only two logical choices) about where he would go, where the damnable universe would send his fledgling excuse for a heart. Option A) The remnant goes to Kingdom Hearts, the coalescence of all hearts within all worlds, bathed in mockingly pale Light; or B) The remnant goes to the Realm of Darkness, which was much more his speed. If you were to ask him this question just days ago, at the pinnacle of his supposed power, he would have told you to shove that goddamn moon up your proverbial ass and send you on your merry way to Hades' seventh circle.
He didn't expect to come here; he didn't expect NOTHING, at least, while still cognizant of what nothing is.
He dodges (to the left? His left.), finding no openings to attack his opponent, he swings his Keyblade wildly in an attempt to just tire his opponent out. He twists and turns with unexpected glee and annoyance; while not really understanding how or why, this fight was more rejuvenating to him than fighting that blue haired bitch and her wannabe child alongside the norted boyfriend.
Actually he did know why, and it was really funny, in a morbid way. He was fighting himself.
Not himself in a practical sense. He wasn't attacking his reflection in the mirror or succumbing to the isolating aura of the Realm of Darkness (which totally DIDN'T grate on him while still alive). Rather it was as though he was looking into what he probably would have become if he agreed with his stupid lookalike and stayed with them: a boy, roughly aged 15 or 16; long, impossibly spiky, jet black hair on that stupid kid's face; and a cold, wide "smile" that would have sent that Neverland cat into therapy. But what startled him about the imposter was his eyes: one eye a brilliant gold, and the other blood red.
"Careful," said the imposter in a neutral tone. "If you don't stop, you will ruin everything."
"What in the motherfuck are you talking about?", he growled with endless venom. "I thought Sora and his band of circus monkeys dealt with you!"
The imposter simply sighed, him watching as the imposter floated his way down onto A PLATFORM THAT APPEARED OUT OF NOWHERE?
"I am not Xehanort," said the imposter, looking forlornly down onto the platform in question and chuckling to himself. "I would have thought you of all people would recognize the sexiest person throughout the worlds!"
Vanitas paused, and then huffed. And huffed again. And again, until he couldn't stop himself, diving into a fit of hysterical laughter.
"Don't you even TRY start on how you're actually me, or some time travelling hindsight enlightened version of me!"
More uncontrollable laughter. "What kind of outfit is that anyway? A pair of KHAKIS?! How dumb do you think I am?"
The imposter once again looked down at the platform, right below where he was just floating for some inane reason. Vanitas gave into temptation and followed suit: a huge circular platform currently was holding them upright amongst the otherwise empty void, emblazoned with colors only attributed to what he understood to be darkness: light, cloudy blues and grays, reds and purples so rich it almost looked like fresh blood, and of course, black. The black he could see was part of an image of himself, with his visage stretched out along the right side of the circle, holding his beloved Keyblade, the Void Gear. He recognized the Keyblade Graveyard in the background, and the insignia of his Unversed strewn about along the circumference. Before he could actually be kind of amazed at it, however, it changed, with the imagery of the Keyblade Graveyard fading away into something like a giant city; his outfit morphing into the exact same as the imposter: grey khakis, a purple and red shirt covered by a leather jacket, and instead of a helmet, he instead wore a beret.
But was bothered Vanitas the most on this mysterious platform were the other people that accompanied him on it. Of course he recognized Ventus, nobody that looks so idiotic can come off that seriously. He saw Xehanort, in all his bald glory, clutching a piece of Vanitas assumed to be the -Blade and smirking with total arrogance. He even recognized the Princess' lookalike, Naminé, but why she was on there and how he knew such information, nobody knows. But the redhaired girl in a strange red outfit he did not know, along with the brunette man in an admittedly stylish black dress shirt, smirking devilishly to the side.
"Don't you see?" the imposter asks, seemingly pleading as if this was the most obvious thing in the nonexistent world they found themselves in. "Your heart shows you the truth, although as I recall by that point you have yet to attempt to listen."
Vanitas was even more enraged than before. Which probably wasn't possible but is somehow happening. First off, fuck this cryptic asshole. Second, if he is just Vanitas from some other time or place, then Vanitas will just take his Keyblade and thrust it into his chest. Right now. No fucking way was he turning into THAT.
"Who are you, really?" snarls Vanitas.
"Even if I told you right now, it wouldn't matter," the imposter reluctantly replied.
"After I am done, you will not remember this ever happening, and you will start a new life, a new journey, of your own free will."
Vanitas was, for the first time in a very long time, scared to see the imposter's face as he said that.
He was happy.
He sounded satisfied.
Vanitas screamed into the void for all it was worth and tried once again to kill the imposter. He lunged fast, closing the distance from above the platform to the imposter in an impossible speed, only to be blocked and sent flying back the way he came with a light based Reflega.
"I do not have time for this," the imposter muttered, starting to look concerned. "You must go, and I must locate her."
"What?! Find who?! Where the hell am I going?! I'M DEAD, you stupid bast-"
The last thing Vanitas could register before losing consciousness was a strange symbol glowing from the platform, and the imposter rushing him with a fist coming right for his face.
"Finally found you," the man said.
"Phi."
The other man, known by many as Van, silently looked down on his past self. It irked him relentlessly that in the past, he dealt with the shitty life he was given and the accompanying trauma by giving in to the old man's plans by lashing out. Didn't matter at what, who, where or when the rage was directed; that was all he knew until this very act, the one he now sees from the other side of the aisle and fully understands. He found himself looking down at his Heart, at his previous Keyblade, Void Gear. He hated that thing, and what the weaponized reflection of his Heart of that time stood for.
He summoned his Keyblade proper and nodded reassuringly to himself. He found Xi and Eta earlier on his journey. Phi is the last, the final piece to the puzzle he has to solve.
To save them all. To save him. To save her. I owe them all that much.
Van summoned the Liminality, and his past self was sent hurdling through. He was about to start reminiscing, although the bright white light of the symbol on his Heart was beginning to grow dimmer.
Time was of the essence. Time to begin.
Van simply points his Keyblade at the swirling symbol, and settles himself, letting the familiar warmth of the light consume him.