When My Hero Academia first began, Midoriya was chosen as the successor to All Might's power, and therefore was set on the path to mastering One For All and becoming the Symbol of Peace. Back then All Might hinted that he was coming to U.A. Academy in order to find a successor, but stumbled on Izuku Midoriya instead. But this plot reared its head once more in the latest episode of the fourth season as fans learned that All Might had originally planned for Mirio Togata, the current top student at U.A., to become his successor.
In the second episode of My Hero Academia's fourth season, Midoriya approaches All Might about getting in touch with his old sidekick, Sir Nighteye, for his work study. But All Might instead has him get in touch with Mirio Togata, and remarks to himself about how Mirio almost inherited One For All.
When All Might asks Mirio -- who is currently on his third year of work study with Sir Nighteye -- to introduce Midoriya to his former sidekick, Mirio asks Midoriya about his dream. Midoriya tells Mirio about his dream of becoming a hero that can protect everyone, and though his dream is complicated, Mirio laughs it off and says that he has no reason to refuse. It's this moment in particular that sparks All Might's memory.
He flashes back to a talk All Might had with the principal, and the principal recommended Mirio Togata to him. The Principal notes that while Mirio's grades aren't great, he has a lot of presence with the other students at the school. Noting that there's always echoes of laughter around him, his smile never fades. Mirio essentially fits All Might's philosophy of saving the world with a smile perfectly, and it's here that All Might thinks that if he hadn't met Midoriya than Mirio would have been the successor.
This is a pretty big bombshell going forward especially because Mirio Togata is about to be a lot more involved with the series. Season 4 features him prominently as the Shie Hassaikai arc puts him in several huge battles, and this will certainly put him in a new light as not only does he have the pressures of being the top student on his back but the original One For All successor choice as well.
My Hero Academia was created by Kohei Horikoshi and has been running in Shueisha's Weekly Shonen Jump since July 2014. The story follows Izuku Midoriya, who lives in a world where everyone has powers, even though he was born without them. Dreaming to become a superhero anyway, he's eventually scouted by the world's best hero All Might and enrolls in a school for professional heroes. The series has been licensed by Viz Media for an English language release since 2015. My Hero Academia will also be launching its second big movie, Heroes Rising, in Japan this December.