You know what quirk would fit izuku well?
Literary materialization, like the Silvertongues from Inkheart.
This quirk enables Izuku’s intelligence to shine more with having a broader amount of knowledge due to being well-read. (He’ll be an even bigger nerd because not only will he be a hero otaku, he will also be a literary otaku.)
Mechanics:
- Silvertongue (quirk name) is a passive ability. Izuku can’t turn it off. It automatically activates when he reads something out loud.
- It’s limited to fiction. He can’t read out things such as “All Might miraculously comes and saves the day” or “Cleopatra rises from the dead” because they’re essentially non-fictional people. Fictional stories about real people (e.g. Magical Girl!Midnight) are also not affected by Izuku’s quirk.
- It doesn’t have to be a book: video game dialogue, movie/play scripts, amateur creative writing, etc… can be affected by his quirk as well.
- He can read out something he wrote himself, but only if he copied it off a book or to add something to the book’s story (e.g. He summoned Smaug, he then writes that the dragon will breathe fire to his enemies). It doesn’t have to be on the book itself; it could be a separate paper or even his own skin.
- The faster he reads the words, the faster it will materialize. The sheer speed of his muttering is a by-product of honing his quirk. (Also, I bet people will be even more creeped out because he’d be a more manic mumbler compared to canon.)
- If he erases/smudges/damages the words or where the words are written, whatever it was that he read out would instantly stop/be gone. He also can’t materialize what he read if he doesn’t understand the words. (A good way of covering this weakness is to learn other languages).
- Overusing his quirk leads to being unable to speak (could be a numb tongue, a hoarse throat, or temporary muteness).
I imagine Izuku would never go anywhere without a pen with washable/disappearing ink and a book or a notebook filled with lines he copied off from a story.
Heck, his arms, legs, and torso could be covered in words or papers with words on them. For sentences that he uses frequently, they could be written on his costume or he can have them permanently tattooed, leaving the rest of his body free to write on.