I'm throwing references from various fan-made crossovers to illustrate that the project, both canonically and rudimentary, accepts all kinds of crossovers. And yeah, this is a worldwide collaboration in making.
By the way, spoilers from included and/or represented works abound. Please read this fiction carefully.
6:40 A.M., morning, day 20. The alliance sets foot on Reality-2147483647. The ships and vehicles and broaded, and the peoples went to the ground, coming to infiltrate everything there.
"We're here for the survivors and the Loop." The Foundation told everyone.
"Kirito, you do not mess up." Roland told Kirito.
"Getting to the highlight, shall we?" Ace Ukiyo as Geats.
"All right, that's a mission for Captain Velvet Meteor!" Captain Velvet Meteor on the place.
"This is the place. Apprehend the objection, and save all!" Terra Branford took note.
"You all saw it coming." Kratos, the God of War, targeted the Sanctum. "If there's any hostile sentry, I'll rip them off!"
"We graced to this day, and is setting in this horizon." Sora evoked.
"All fates converge..." System No. XX. "Stand tall, and shake the heavens."
Everyone is here. The entire alliance and their recruits across all those worlds, across so many timelines, paradoxes included, and across dimensions and realities, whatever survived the destruction of the Zero Point and the consequences afterwards. Everyone there, most of those imaginable, are all here. Every mechas no bigger than buildings, too.
Mitra is holding an Olympic Torch. Specifically, the Tokyo 2020 Olympic Torch.
"Hell comes loose..." James Bond holds the London 2012 Olympic Torch. "No time to die."
Various warriors is holding the Olympic Torches from across the history, both (modern) summer and winter games' relays. (David and the Origin, in particular, are holding the Sydney 2000 and Athens 2004, respectively.) The torches aren't lit with normal flame however, but with the flame from the Last Reality like that of the Origin.
Cross-trivia: The Olympic flameis a symbol used in the Olympic movement. It is also a symbol of continuity between ancient and modern games. Several months before the Olympic Games, the Olympic flame is lit at Olympia, Greece. This ceremony starts the Olympic torch relay, which formally ends with the lighting of the Olympic cauldron during the opening ceremony of the Olympic Games. The flame then continues to burn in the cauldron for the duration of the Games, until it is extinguished during the Olympic closing ceremony.
An army of sentries appeared, defending the Sanctum.
"Here they comes." Reiji noticed.
"I. AM. THE. CHOSEN. ONE. ALL MATTERS IN ABSOLUTE ONE. THESE MIGHTY IDEALS IS MY PARTS. WHEN I COMMAND ENEMIES' SURRENDER, I SPEAK WITH INFINITE VOICES... WHEN I MAKE A FIST TO CRUSH ENEMIES' RESISTANCE, IT IS WITH INFINITE DIMENSIONS! WHEN I STARE INTO EMINENCES ENEMIES' EYES, AND SHATTER THEIR DREAMS. AND BREAK THEIR HEARTS, IT IS WITH INFINITE WORLDS! NOTHING LIKE ABSOLUTE HAS EVER COME AMONG YOU: NOTHING WILL AGAIN. I WILL TAKE MY ENEMIES TO A HELL WITHOUT EXIT OR END. AND THERE I WILL MURDER THEIR SOULS!
AND MAKE THEM CRAWL AND BEG! AND DIE! DIE! DIE FOR ME!"
"They're speaking through... the voice of Empress Zero!?" Daxter is shocked.
"We got millions plus, how many do they have?" K.O. looked around.
"All multiverses ready, all amalgams ready, all variants ready..." Axel Asher (Access) listed.
"We're gonna getting stronger, even after all of this!" Vegeta told everyone. "Only weakly cowards would put an absolute limit on their powers for no reason!"
"Die! DIE!" The sentries and some bigger monsters attacked.
The entire alliance warred with the sentries, and fought like the great soliders and warriors or old, but boosted. It get bloody from there on out: in particular, the Avengers (both teams of that name, plus various variants), the Justice League (and its variants), Spider-Man (across the Spider-Verse), the Fantastic Four, the X-Men, the Eternals, (all versions of) the Teen Titans, the Doctor (all incarnations), The A-Team, Sherlock Holmes (and all other incarnations), the Ghostbusters (plus both incarnations of the unrelated (Filmation) team), He-Man, She-Ra (all incarnations), the Scoobies, Mystery Inc., RoboCop, G.I. Joe, The Autobots (and all subsequent incarnations), the BPRD, Stargate SG-1, Battlestar Galactica (and the original), every single Power Rangers team (so far), Matthias and Martin the Warrior, the Pan-Pacific Defense Corps, the crew of the Serenity, the Clans, Starfleet, the Rebel Alliance, and so much more, all shed blood just to kill the army.
Less than ten minutes into the war, System No. XX activated her function, aided by Raharu pushing the N.O., and spawned various more...
"Ask yourselves, 'Are you willing to zero out for the omniverse itself? Are you willing to fight for all?'" System No. XX yelled. "In the name of James Halliday, Umbras and Zero Point...Help us save the reality!"
And so, the soldiers across both World Wars, the OASIS (players) and the rest of J-Force, who all has been in reserves up to that point, are brought to the war, adding to alliance's number to go against the sentries and the big monsters. The war got bigger than before long, and blood eventually seen everywhere in the battlefield. Some has to do something to revive any fallen men.
Ikue Higashi and the Chrome magical girl (yeah, similar to Herald) watched the entire war, and is routing for the sentries to deal with the alliance altogether. However, David and Lucy managed to spot them both.
"Those magical girls... The same one we saw in Aionios!" David recognized them. "Lucy, call the Pretty Cures, these fives, Nanoha, Fate, Miss Kinomoto, Ruru,Nowel, Miss Inverse and Hoshisora!"
Lucy told through Akasha Terminal what to do, and even told whoever nearby to protect.
"Sorry, Mr. Geralt, but I have to go." Akemi Homura then followed David, leaving Geralt of Rivia.
"Sorry, I have to hurry!" Sakura Kinomoto ran through.
The magical girls David commanded to be all arrived, and David quickly pointed out where both rogue magical girls are.
"Damn it!" The Chrome magical girl is frustrated. "They're going after us!"
The magical girls approached Cure Union and the Chrome magical girl, leading to a chase. Both even considered teleporting, but they suddenly realized that David already told Dr. Manhattan to watch out.
Even worse, Ryu Hayabusa, Plachta, Opoona and William Adams are teleported by Dr. Manhattan. They blocked both rogue magical girls.
"Ugh!" The Chrome magical girl felt the dead end.
"Just a Cure doing what I can!" Union straight out attacked three foursome.
Being a Cure, secretly Fusion, and (currently) a servant of Genjitsu-no-Omnia, Union managed to fend off Ryu Hayabusa, Plachta, Opoona and William Adams all at once without too much effort, protecting the Chrome magical girl, which could allow her to teleport both rogues out of the way and into the safety. However, Dr. Manhattan and the magical girls sent by David all appeared.
"What's with you, Union!?" Sakagami Ayumi as Cure Echo, confronting Union. "Have you alway been out of your mind!?"
"Magical girls like us are meant to protect, not hurt for no reason." Madoka Kaname told the rogues.
"Therefore, we as magical girls fight to protect whatever they knows they holds dear!" Ruru Gerard argued. "Even though not all magical girls are the same..."
"Ikue, you either protect as a hero, or refuge like a villain!" Lina Inverse yelled.
"Stop, Herald and her companion, stop." Plachta interrupted.
The Chrome magical girl is totally shocked and afraid of this.
"What?" Union isn't amused. "Hmph."
"Actually, my hypothesis turns out to be correctly true for some time ago." Dr. Manhattan unveiled. "Whatever this mysterious magical girl is, she's too similar to the Herald who once encouraged spreading Chrome nefariously. And with the same voice, in fact."
"And as for you, Ikue Higashi..." David appeared, having used Sandevistan to approach Union. "You're the one we were investigating, so don't try to hide anything."
Union is shocked by the fact, and quickly enraged!
"What have you done to mess with me!?" Cure Union is crazy. "That's it! I'm gonna kill you, starting with those 'Pretty Cure'!"
Union went blitzed straight towards the Pretty Cures, and attacked and evaded with killing intent. Because of this, the Cures ended getting wounded and fractured, and struggled to keep up with the actually-evil (and non-existant) Cure.
"You dare defy me... I shall bring reckoning!" The Chrome magical girl quickly summoned Omne, and then teleported away. Dr. Manhattan chased through teleportation.
Omne rises up from the Chrome again, and is ready to crush the members and soliders of the alliance.
"What's this!?" Noctis is shocked.
"So huge!" Zidane (a fellow Warrior of Light) is surprised.
"This isn't good!" Cloud Strife realized. "Watch out!"
"Uh oh... That's getting ridiculous..." Ultraman Zero, even in his typical size (49 Meyers tall), looked at (chromified) Omne.
Various other Ultras, including the Brothers, Tiga, Dyna, Cosmos, Justice, Nexus, Mebius, New Generation, Z, Trigger and Decker, all gasped at Omne.
"What the actual hell is this!?" Ultraman Geed is shocked.
"So big and so much Chrome, how about we go strongest?" Mercurius suggested.
"Fine, dude! Just heal us up to sustain the hell out of our forms!" Ultraman Zero responded.
The Ultras became into Glitter Tiga, Mebius Infinity, Noa, Legend, Saga (by taking Zero and Cosmos, courtesy of Dyna using the Zero Point crystal), Reiga, Z Deathcium Rise Claw, Glitter Trigger, Decker Dynamic Type, and Ultimate Shining Zero. Ohma Zi-O has more idea: summon Zero Beyond to aid them all.
Mercurius looped the lights, allowing the Ultras to fight indefinitely as long as the spell is active.
"Thanks, Mr. Mercurial Snake!" Ultraman Zero thanked Mercurius. "Make sure Omne doesn't try to attack anyone other than us!"
The Ultras then flew up and bombarded Omne. Meanwhile, the Pretty Cures is slowly closer and closer to death, yet still managed to actually hit off the still-rampaging Union.
"I'm impressed you hold me off as well as you just did." Union smirked. "However, I can break you, and then be able to kill you!"
"Stop it!" Kiana Kaslana interrupted.
"Who're you!?" Echo, the original duo, and the Hugtto Cures has no idea.
"You wretched Kiana Kaslana... you dare interrupt me!" Union is frustrated.
Both Domain of Incandescence and Abyss Flower agitated, and Kiana clashed with Union.
"Finish her, now!" Kiana told the Pretty Cures.
Wasting no more time, the original duo, Nagisa Misumi (Cure Black) and Honoka Yukishiro (Cure White) then ran and physical struck Union together, stunning Union. The rest of the Cures got up, and transformed into their strongest form. Heartcatch in particular manages to become Infinity Silhouette in human size, much to the Cures' surprise.
"Combining our strongest attacks into something to... kill that!?" The Cures are shocked.
"There's no other way!" Nagisa/Black replied. "If we don't do this in time, she could counter the hell out of us, and we might all die!"
The Cures quickly overclocked themselves using the Prism Flower and then some, adding the angelic extension not seen in a long time. The Cures and the fairies leaped up and floated up, with the respective teams and Echo making a circular formation, emulating Rainbow Jewel Solution. Suddenly, each portal, behind each team, appeared suddenly, and behind are the worlds the portals pointed to all became Pretty Cures, cheering 'Hooray, Hooray, Pretty Cure!' loudly.
"Combine our most powerful attacks, and turns it into..." Nono Hana, as Cure Yell.
"Ugh...! I won't allow you!" Cure Union got up. "I have the power of all Pretty Cures!"
The darkness inside mixed with the lights Union normally embodies, and it reminded the early Cures of Fusion. David just started recording it all, and found the truth.
"So that Ikue Higashi is actually Fusion all along...!" David realized. "Ikue Higashi never really existed, after all!"
"Oh... Just a combination of all Cures..." Ikue Higashi, Cure Union, exposed. "Then I will destroy you all!"
The Cures quickly aided by themselves from various points in time (emerged from the said portals) and all their fairies as well. They all simultaneously did their strongest attacks beamed and grouped together into a blinding rainbow beam, whule Fusion, taking the powers of both Cute Union and all the remnants of every single foes all the said Cures faced up until this point, fired a massive, red, black and pink beam. Both those massive beams directly collided with each other, creating a shocking burst that released so much energy, it shaken the ground of the battlefield and rippled the Chrome, all nearby. It can even be felt away...
"Unbelievable! The purifying energy is closing with the corrupting energy, and that's shaking things!" Lucy noticed.
"So powerful!" David responded. "Run, Lucy! Run!"
The beams clashed for nearly a full minute, and Fusion's beam slowly started overpowering the Pretty Cures.
"Die!!!" Fusion sheds the Ikue Higashi identity and matters, discarding the rest of her existence for its own survival.
"Wait... what?" David is surprised. "How did Fusion...!"
Fusion's beam is slowly overpowering the Cures, and if it completely destroy their beam and reaches them, they'll be annihilated, dying from the corruptive burst. The Cures is currently around the limits, but they're clearly pushing it, and thus, they all coughed blood. However...
"Cure Rainbow... Mother Heart...!" Echo struggled.
Cure Rainbow and Mother Heart appeared, and embraced the Cures, giving them an insurmountable boost to power, whule at the same time, shedding Ikue Higashi overpowered up Fusion's beam, which would've been enough to wipe out the beam and thus the Cures in an instant. But the Cures' beam, thanks to the embracing boost, instantly surpassed all that, and overpowered Fusion's beam to the end, finally wiping it out.
"Nooooo!!!!!!!!!!" Fusion is engulfed by the beam.
The beam barely misses David and Lucy, and it's so powerful, it tore a hole in the way, rippling out the Chrome around, shaking much of the Sanctum. Everyone in the battlefield suddenly noticed it all.
"Pretty Cure!?" The alliance is surprised.
"Ikue Higashi we were investigating is actually Fusion, and the Pretty Cures just wiped it out!"
To be continued...