Based on a concept by Nobodyfamousposts on tumblr that I used with permission and it based on a prompt I gave. I am using concepts from Endgame so slight spoilers.
The 'Decimation', that is what they are calling it. Apparently it started with an alien attack in New York and it led to a battle in the African nation of Wakanda. Apparently it was all masterminded by this purple alien warlord called Thanos.
Everyone though the Avengers would be able to take care of it; despite the Sokovia Accords they thought they could come together again to stop this.
However they failed; Thanos got what he was after and was able to wipe out half of all life in the universe.
Marinette had been patrolling as Ladybug along with Cat Noir when they saw it start to happen; people began turning to dust and there was fear of an Akuma attack. She went to Cat Noir to try and regroup but was too late.
She had arrived to find a pile of dust next to Cat Noir's ring with Plagg crying over it. She comforted the Kwami as they dealt with their shock and heartbreak and she picked up the Ring.
She did not know what to do and tried rushing home to check on her parents and her biggest heartbreak was finding two piles of dust. She 'bugged out' and cried over the piles as Tikki and Plagg did their best to comfort her.
She was unable to cope with anything and was too heartbroken to try and return to her home after that. She was basically homeless and was out and about trying to help the equally confused and heartbroken people as best as she could. She did not dare try and look for anyone she knew at school in fear of further heartbreak.
The best she was able to do was try and find answers from Master Fu but one day soon they were approached by a man called Wong. Apparently he was a Master of the Mystic Arts and was working with the Avengers on a plan that needed their help.
Apparently the power of the Miraculous was about as great as the combined 6 Infinity Stones which was what allowed Thanos the means to carry out the Decimation. They told her they needed her help to put together a plan to create a Miraculous Wave to fix everything THROUGH THE ENTIRE UNIVERSE!
It was a lot of pressure and a lot to take in and she was not sure she could do what would be expected from her. Master Fu had faith and so did Tikki; it was surprising when Wong brought Captain America himself Steve Rogers to help try and assure her.
It was amazing to hear a prep talk from Captain America himself; she was French but she had to admire the WWII hero and leader of the Avengers and it felt like his voice could command the gods.
She still had doubts but felt better knowing legendary Superheroes had faith in her.
Word spread that Ladybug was working with the Avengers to fix things and it put hope and optimism into the minds of the French people. Apparently word got out to someone as well.
She was out on patrol to clear her head when she came face to face with someone she had not seen since Heroes Day. Hawk Moth was there in person and she went to confront him with a glare but was surprised by how subdued and sullen he was. "You will need the might of all the Miraculous to do this so I want to call a temporary truce. Here is my parting gift." Hawk Moth told her calmly as she was then handed the Peacock Miraculous itself.
Did Hawk Moth suffer through this as well; she was hesitant when he held out a hand to shake. Hesitantly she ultimately shook it.
It took a month to get everything together; the Avengers were given a Miraculous to use for this. Tony Stark who was returned from space apparently was able to help while recovering from his ordeal. Bruce Banner and a talking alien racoon called Rocket (Marinette did not even want to question it) were in charge of the technology aspect. Wong was leading the Masters of the Mystic Arts and some Asgardian sorcerers (helping on behalf of Thor who was there) for the mystical parts.
A part of her was excited to be working with the Avengers but she was mostly focused on everything.
They had help in some sort of flying space heroine called Carol Danvers who apparently left earth during the 1990s. They were working at the Avengers Facility itself where she was staying for the time being. They got parts from old SHIELD bases to help construct everything.
They found a cat at Fury's old office at the Triskellion called Goose who Carol seemed to know; Marinette could not help but started fawning over her. She then pretty much became Goose' caretaker.
After a month everything was set up and she looked to Hawk Moth and the Avengers (and a cyborg friend of Rocket's called Nebula) who were using the Miraculous and names she had given them. She then cleared her breath and concentrated as everything was set up.
Thinking of everyone who was counting on her she then felt the power of all the magic and Miraculous coursing through her. Giving it all she could she then threw her Miraculous Yo-Yo into the air shouting "MIRACULOUS LADYBUG!"
She then saw the wave of Ladybugs flying through the air and going through the skies. She watched in awe with everyone else. Then suddenly they saw people starting to appear and the skies becoming brighter.
Steve saw his friends Sam Wilson and Bucky Barnes returned in the Facility to which he greeted them in a hug.
Clint Barton had his phone in his pocket while he was watching the Ladybugs. He took it out and was practically in tears of joy when he saw it was his wife Lila calling.
Thor had Stormbreaker in hand and found his phone calling and it was from Valkyrie; she had been setting up a New Asgard in Norway with land that was given to them. "Thor; I don't know how to tell you this but among the refugees... I am seeing people who were butchered when Thanos came and I better give this call to someone else."
Thor waited silently and was ready to leap in joy when he heard Loki's voice tell him "Thor what is going on? I am pretty sure where I am that this is not Valhalla..."
All the way in Wakanda Queen Ramonda was deep in mourning for her children when she was looking out and saw the Ladybugs. She then saw the Wakandans returning on the sky and looked behind them and only glanced at T'Challa and Shuri for a second before embracing them.
Wong smiled as he saw Stephen Strange returned.
Wanda was watching and felt a hand on her shoulder. She turned around and embraced the Vision who embraced back.
Marinette saw Peter Parker in his Spider-Man armor appear out in the field and remembered what Tony had told her about him. She let out a hand to help the fellow red and blue Hero up.
"Am I in Heaven because if so where is my Uncle Ben and who does an Angel look like a girl superhero?" Peter asked and Marinette gave a laugh at him; she thought she would get along fine with him. She then felt her earrings flashing and turned back into Marinette.
"You returned from life and since i know your secret identity since Mr Stark told me... Marinette Dupain-Cheng." Marinette told him as she helped them up and they shared a smile.
Tony was watching this from a window of his bed where he was healing. He was watching with Pepper as he got ideas on setting his protégée up on a date with a French superheroine.
May Parker had been staying at the Facility with Pepper setting her up for classes to try and help with her grief while they work on bringing her nephew and everyone else back. She rushed down the stairs ready to embrace her nephew.
Hawk Moth was watching and when he saw Ladybug had her earrings flashing she resisted all desire to watch and learn the secret identity of his nemesis. He faced away from her and had Wong take him back to France. He had a son to see.
Rocket and Nebula heard on the communication device to Carol who was in space to look over the Wave. She then told them "I see the wave of Ladybugs... they are going further... higher... faster..."
Rocket and Nebula saw Peter Quill, Gamora, Groot, Mantis and Drax and they were more than happy to embrace them. Gamora was happy to embrace Nebula once she recognised her.
"I think you should explain what happened Nebula; whatever the story I believe it will be the story of all stories." Drax asked to which everyone agreed.
"First things first." Gamora spoke up and she then faced Peter Quill who was ecstatic to see Gamora alive again. "Peter, I have been in the Soul Stone and I saw everything that Thanos had done and everything else that happened nearby." Gamora explained and then she then kicked him in the unmentionable areas twice. Everyone laugh and smiled as Gamora angrily shouted at him for being an idiot.
On his peaceful planet Thanos was enjoying his retirement; using the Stones he had created a huge garden for him to spend all his remaining time as a farmer.
He had done it and despite all the pain, the sacrifices and heartbreak he had done what he had set out to do. He had used the Infinity Stones to balance the entire universe. He knew what would happen now:
He would spend the rest of his life as a humble farmer while the rest of the universe flourished thanks to him. He would look to the sky and smiled thinking of the grateful universe looking to him as their hero. He would be celebrated as the man who had saved the Universe from itself.
They would make statues; monuments and holidays celebrating him. then he would pass on he would join the lost citizens of Titan in the afterlife. They would welcome him as a hero and apologise for calling him mad after they realised he was right all along; his father A'Lars would embrace him and beg for his forgiveness.
He would be more than happy to do so and he would forgive their short sightedness. From his rightful place in the afterlife he would look on as he watched the peaceful universe that he had created.
He had gone out to continue his farming looking to the sky and was confused when he saw a wave of red filly the sky. What was that?
As far as he knew nothing in the universe could cause that. Then he was met with the Stonekeeper from Vormir. "A soul for the Soul Stone; since the Soul has been returned so has the Stone." He said simply as he then pointed to the wrecked Infinity Gauntlet that was now permanently on his hand.
He saw the Soul Stone disappear and then the Stonekeeper. His confusion grew as he then saw the Mind Stone disappearing as well.
It took a moment to process this as his mind finally found what must have happened:
Somehow, someway... someone had undid what he had done...
The Infinity Stones were the most powerful force in the universe so who or what could have had the power to reverse what he had done? Plus what ungrateful soul would dare even try?
He then let out a screaming of primal fury. He then went to his old armor he was using as a scarecrow. Luckily he had yet to destroy the 6 Infinity Stones otherwise he would be unable to use them to do this one last mission.
He still had work to do to fix what he had done; whoever had reversed what he had done will pay dearly for what they had dared to do...