AN: This is an idea I recently had that I WISH I'd thought of in time to put on the poll for chaptered stories. I really think this concept would make an amazing story so what I want you all to do is this. Read the chapter and if you like the concept, go find your other favorite writers for the My Hero Academia fandom and have them come read this chapter, ask them to pick up the idea. I'm willing to let it be adopted so long as they clear it with me and agree not to do certain things I really don't want (such as certain pairing choices, etc.)
AN2: This story has a smarter Shigaraki, taking advantage of All Might's many faults. And I really do think how I'm showing this would be in character as 1. Izuku is a hero worshiping kid who's known All Might for barely a month at this point when my story starts. That would mean much more to him than it would to All Might who is used to people seeing him in that light and one month to him carries much less meaning than it would to a fourteen-year-old. Hell, we know after the entrance exam he ignored all of Izuku's calls and messages. He claimed he was busy, but he couldn't have taken five seconds at the end of the day to send a text or a quick call saying 'hey, sorry, real busy at the moment so I won't be able to talk for a week or so'. Plus, we see how quickly he changes his opinions on Shigaraki in the show the moment he learns of the psycho's bloodline, so I really do think this is all in character.
AN3: This isn't a polished first chapter but several pieces that show the intent and concept behind the story.
All for One or One for All
Frowning, Izuku checked his phone again. All Might was almost an hour late at this point to their training and he hadn't heard of any villain attacks. "I…I'm probably just overthinking it." Izuku shook his head, trying not to feel hurt or worried, "He probably just got called in for some emergency that needs secrecy. Yeah, that has to be it." clenching his fists, he went back to clearing the beach, continuing the grueling routine that All Might had set him on to be ready to inherit his power, the sacred torch One for All.
"Is he all right?" All Might demanded from the doctor as he stood outside a hospital room, the patient inside being someone he hadn't known existed until just recently. Getting a call from a hospital that someone named Tenko Shimura had been found and was asking for him was a shock, but he'd run right over regardless. He couldn't abandon his Master's family.
"He will be." The doctor assured, the man clearly trying to keep his calm around the 'Symbol of Peace' "But he's definitely shaken. My biggest concern is his quirk. He said his quirk was a disintegration one, but it won't activate, and he shows no sign of quirk factor even though his toe only has the one joint."
Paling, All Might nodded, stepping in as the Doctors left "Hello young man." He stared down at the pale form of Tenko Shimura, the grandson of his own beloved mentor Nana.
"All…Might." The man looked up at him with frustrated eyes, his voice dry and raspy "He took it All Might, he took my quirk."
"All for One." All Might clenched his fist angrily, how dare that monster rob Nana's grandson of his quirk? How dare he?!
"What am I supposed to do now?" Tenko whispered "That quirk was my one ticket to a good life. I don't have a degree, I don't have any money, or a home. What am I supposed to do now?" hearing the broken young man, seeing his chances of being a hero crushed, All Might quickly came to his decision. He knew he'd been training young Midoriya, but that boy wasn't the only worthy one around. Not to mention he grew up quirkless while Tenko here hadn't experienced it before and had nothing else to fall back on. No, it was a sign, it was fate. He knew what he needed to do.
"Young man, what if I told you I could help?" All Might smiled to the young man as he studied him, Nana's grandson clearly already possessing a body capable of inheriting this awesome power, "That I could grant unto you a new quirk, one that was granted unto me by your Grandmother Nana."
"That…that's possible?" Tenko looked up at him in awe and hope as All Might smiled, plucking out a hair.
"Yes. Yes, it is my young pupil. Eat this, and my power shall transfer to you. You shall inherit my strength and become the next symbol of peace." All Might beamed like the sun as the young man took the hair, swallowing it with a loud gulp before making a face at the taste that had All Might chuckling.
"So…I have your quirk now?" Tenko asked, flexing his fingers in confusion as if expecting something to happen straight away.
"Not just yet, the hair has to digest first, but I'd say within an hour or two you'll be right as rain." All Might laughed happily, Tenko joining in only for his laugh to start becoming twisted, something dark sounding.
"Oh, that was too fucking easy." Tenko cackled as he climbed out of bed, a misty portal appearing in the room, a gas filing out as Tenko put on a mask he pulled from the portal, "Thanks for the Quirk All Might, but I have to say, never call me a Shimura again. I am Tomura, Tomura Shigaraki. And my master will be so pleased that the quirk he created is back to the side it belongs." The man cackled, stepping through the portal even as the gas had All Might paralyzed and unable to move. What just happened, what had he done?
Still laughing, Shigaraki walked to the lab where his master was, as well as that 'Midoriya' kid that All Might was originally going to give the quirk to. "Well done my young pupil." The voice of his sensei called out to him pleased as he entered the room, a screen showing the hospital room he'd just left as All Might fought to overcome the dissipating gas, "I must say I was impressed your plan went so smoothly, not to mention your thought of bringing the Midoriya boy here to witness All Might's betrayal, simply inspired."
"Not to mention what an excellent find he was." The 'Doctor' cackled as he gestured to the tank where that Midoriya kid was floating, eyes wide with pain and betrayal as he stared at the screen, body spasming in the fluid he was submerged in.
"What do you mean doctor?" Sensei asked, apparently not knowing what the man meant either.
"I injected him with your quirk factor as instructed and got a new result. Before we only ever saw three results, that being nothing would happen, the quirk factor would reject violently to kill or cripple the host, or the host would get some degree of your quirk to the point they could take a certain number of new quirks. This boy though has formed something new. His quirk is almost identical to yours apart from needing to 'build' his reserve like one would a muscle. Once his body gets used to having so many quirks he can take more, and more. He can take and give just as you do."
"Excellent!" Sensei laughed, "Oh, what a beautiful find you and our 'dear' All Might have given me. Regardless, come here Tomura, it's time for me to return Disintegrate."
"Yes master." Tomura eagerly hurried over, his master placing his hand on Tomura, thankfully without Disintegrate active, and pushed. Instead of the feeling of his birth quirk returning to him, Tomura felt a resistance, a barrier. "What?!"
"It would seem One for All does not allow new quirks to be added to it." Sensei frowned, sounding genuinely surprised "I am sorry Tomura."
"It would still make an excellent quirk for a Nomu." The Doctor offered, not even phased by the fact that Tomura couldn't have his original quirk back.
"At least your new power is much stronger." Sensei chuckled "And you won't have to worry about destroying as many of your possessions. And now I can have three powerful villains to run and inherit my empire. You shall lead them, we shall find another to take my quirk, and when we break this boy you shall have a third."
"Yes master, it shall be done." Tomura bowed, still processing that disintegrate would never be his again.
"Kurogiri, take Tomura back to his home so he can finish absorbing that blond ape's quirk." Sensei ordered as a portal appeared, Tomura dutifully stepping through.
"Tsukauchi, you said you found him?!" All Might panted as he burst into the precinct, the past month having been hard on him when he was forced to tell those in the know of One for All what had happened. They were…less than pleased.
"Yes." His old friend nodded with a wince, "He's with his mother now giving his statement. But I have to warn you Yagi, he doesn't want to see you. Hell, I'd say it wouldn't be a stretch to say he hates you now. Apparently, the ones who kidnapped him were All for One and his group, they had a video feed of that hospital room so that boy saw what you did. He feels betrayed and I don't rightly blame him. With his new quirk I'm just glad he still wants to be a hero."
"Wait…new quirk?" All Might paled at the implications.
"All for One has been experimenting on injecting others with his quirk factor and it took near flawlessly into Izuku Midoriya. He has to expand his 'reserve' he stores quirks in but there's no upper limit. It's how he got out, using his meal break to steal the quirks of a couple there he'd observed, apparently working for that monster in the promise of stronger ones, and combined them to get out. It was volatile and drew attention, but it got him out and got him to us. Sadly, that base was already cleared out by the time to heroes and police were organized enough."
"So…what are you suggesting I do?" All Might frowned "I was training him. And I can't just let someone with that power run around without supervision. You know who wielded it first, that power is evil and corrupting by nature!"
"All Might, that's not really your decision anymore."