Theory-Midoriya actually has a hidden quirk(minor spoiler)

Much like the younger brother of All for One, Deku is not quirk-less but had a well-hidden quirk no one knew about because it is virtually useless by itself. His is the power to boost quirks exponentially. Since Deku inherited no secondary quirk, it couldn't manifest. One for All once it was passed to him is evidently dangerous, so dangerous the recoil breaks bones, tears muscles, it risks not only disfiguring the hero, but blowing off the limbs of the user. All-Might never had to cope with such deadly drawbacks. Deku is boosting the stockpiling quirk to insane levels, where he can speak to the previous seven deceased torch bearers and use their quirks as his own.