oKAY but what if afo had all might followed pre-series and izuku was kidnapped right before all might showed up to the beavh on the day izuku gets ofa? and the last thing he hears is a scream for help, and all that is left of izuku are his shirt, shoes, and backpack, still left on the staircase

tahwraven:

Okay Anon, let’s Do This.

• All for One has people
  investigate All Might after he’s recovered (as much as he can) from
  their fight. He finds out All Might’s identity and has his people keep
  track of Yagi Toshinori.
• He doesn’t do anything about the
  information he gathers from this just yet. He likes to torment his
  brother’s successors, so he plans to wait for the chance to inflict
  maximum agony.
• The morning of the U.A. entrance exam, Toshinori
  reaches the beach planning to give Izuku One for All. He arrives just in
  time to hear Izuku scream. All that’s left of Izuku is his shirt,
  shoes, and backpack.
• All for One has been wanting to experiment
  with the Plus Alpha aspect, to find out why some people have quirks and
  some don’t, but with the fall in population of quirkless people, it’s
  hard to find anyone to experiment on that won’t draw attention. How
  timely then, that he wants Izuku’s disappearance to be noticed.
• This
  isn’t why he kidnapped Izuku – he did that because he wants Toshinori
  to suffer and would have done it even if Izuku wasn’t quirkless – but it
  sure is a convenient opportunity.
• He also doesn’t just do
  experiments. Izuku has not been officially made the ninth wielder of One
  for All, but he was chosen as next in line, so he still counts. All for
  One wants him to suffer.
• The experiments have an
  interesting result. He awakens Izuku’s dormant quirk, the ability to
  manipulate the temperature of things.
• Izuku has difficulty
  controlling it, having never used a quirk before, and any control he has
  is shot to all hell when the experiments continue.
• Shigaraki
  doesn’t think anything of the kid Kurogiri brings in. Usually, it’s
  adults, but it isn’t always, and this happens all the time.
• But
  Izuku is terrified he’s going insane, driven by being experimented on by
  the good doctor and otherwise having little human contact. Isolation
  leads to him talking to the one person around who hasn’t shown him much
  attention.
• They talk philosophy. It’s weird.
• Shigaraki
  doesn’t outright state why he hates All Might, but what he does say is
  telling enough. Izuku lets him know that heroes are great but “they’re
  still human. They can’t save everyone.”
• “You know that means it’s likely they won’t save you?”
• Izuku
  shrugs. He’s considered that. He lost the expectation that someone
  would come save him a long time ago. (No one cares about a quirkless
  person anyway.)
• “Carrying around corpse hands isn’t a thing people do.”
• “The chemicals used to keep those from decomposing is unhealthy to breath in. I can smell it from here.”
• “You should use some chapstick.”
• During
  this time, Izuku sees several other people brought in and experimented
  on. Most of them die. One, he sees turn into a noumu.
• Shigaraki
  mentions Tsubasa as the last kid to be brought in. He didn’t know that
  Izuku knew Tsubasa, and doesn’t understand why Izuku cares about some
  kid he hasn’t even seen in years.
• When Izuku gets the full story
  out of Shigaraki, he wonders with disgust why All for One kept his
  father’s hand before Shigaraki decided to carry it around with him.
• Izuku trying to teach Shigaraki how to human.
• In
  the meantime, Toshinori is trying to find out who took Izuku, but there
  aren’t any leads. He and Naomasa often visit the Midoriya household to
  check up on Inko. Toshinori doesn’t tell her who he is, but he explains
  that he’d been helping Izuku train and was the one to witness Izuku
  being kidnapped.
• Aizawa is disdainful and longsuffering when
  it comes to this man who strives for the spotlight but simultaneously
  tries to hide. Then he sees Toshinori looking tired and depressed all
  the time, not as boisterous as he usually is. It only seems to get worse
  as the weeks pass by.
• Toshinori is trying his best to teach but it’s hard when everything he does feels lackluster. The other teachers worry.
• Since
  he’s so focused on finding Izuku, he’s less so on hero work, so he
  doesn’t overwork himself on the day class 1-A heads to U.S.J. He’s at
  the facility from the beginning.
• It goes mostly the same, but
  Shigaraki is somewhat unsure of himself. All Might is just like what
  Izuku told him he was: stronger in personality than in body, intent on
  helping everyone he can, and blames himself for any perceived failure.
  This isn’t the musclehead hero he’d grown to hate.
• Shigaraki’s
  growing sympathy for Izuku leads him to send a lackey on a mission – the
  lackey leads the police and heroes right to the warehouse in Kamino
  where Izuku is being held.
• Eraserhead is part of the party that
  raids the place. He and Gang Orca are the first to find Izuku. He’s
  naturally traumatized and doesn’t say anything. They don’t know who he
  is until Naomasa sees him and recognizes him from the pictures Toshinori
  showed him.
• Aizawa and Sakamata witness Toshinori’s reunion with Izuku, and Aizawa realizes why All Might has been so despondent lately.
• Toshinori finds out All for One is alive.
• Nedzu meets Izuku and plans to give him a chance to get into U.A., but right now he needs to recover.
• Izuku is not okay.
• He
  feels jittery and panics easily when he remains at home with nothing to
  do. He is unable to relax without Toshinori or, to a lesser extent,
  Aizawa or Sakamata nearby, so he often ends up following them around. He
  assists Sakamata in general office work but usually accompanies Aizawa
  or Toshinori to school.

BONUS:

• Bakugou had,
  when Izuku first went missing, claimed he’d just chickened out of the
  exam and that he’d turn up when he felt like he could show his face
  again. A week later, Izuku is still missing and Inko is at his house
  while Mitsuki and Masaru try to keep her calm.
• He thought he
  felt powerless when he was attacked by that slime villain but this – no
  one even knows what really happened and there’s nothing he can do.
• He
  avoids the house when Inko is over, caught between that helplessness
  and the shame of having bullied her son, so he doesn’t hear about it
  when Izuku is found.
• He is therefore very shocked to see Izuku follow Aizawa through the door into class one day.

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With many thanks to @thenightisdarkandfullofbooty for headcanoning with me.