What about a BNHA Fantasy AU?
Series: Boku no Hero Academia/My Hero Academia AU Criteria: BNHA Fantasy AU AU title (optional): Fantasy!BNHA
I apologize if you were looking for me to take Horikoshi's fantasy AU and run with it. I actually think that going Low Fantasy, where it is after the fall of a high fantasy setting may work better.
300 years before the start of the series, a power hungry demon attacked the lands, and destroyed much of civilization, only contained by two brother mages that worked in unison to trap the demon and seal it away. Both of them were given lands and the people lauded them with accolades and cheers. However, the elder brother grew to desire more and more power, while his brother wanted for the lands and its people to be cared for more. Taking the gem that the demon was bouund in, the mage that would become known as All For One worked to take the demon's power for its own. The brother, bearing the sword that would be known as One For All, confronted his brother with many paladins and knights. As he held the blade ready to deal the final blow, the younger brother chose mercy and instead sealed away a part of his older brother's power in the sword. Years later, the brother was killed by an assassin, but the blade could not be found. As All For One's power grew again, he still searched for the blade, which would almost always turn up in the hands of the next hero of the people to challenge his rule. Magic is rare, and carefully monitored by the forces of AFO.
Izuku Midoriya, the son of a single mother who does alchemy work for their village, is out collecting ingredients when he finds a slime creature. Trying to escape, he is captured and saved by an aged and injured knight, named by the bards as All Might, who had faded into legends years ago. Toshinori Yagi, All Might's real name, banishes the slime demon and falls over. Taken back to his home, Izuku and Inko tend to the man.
Sadly a few days later, the armies of AFO attacks the village and kills everyone but Toshinori, Inko, and Izuku. Izuku was out in the field again, and missed while he was in the forest. Inko was taken with Toshinori, since her alchemical skills were enough to help reverse some of the demonic damage done to his body. Curiously, the blade was missed, and Izuku finds it in the wreckage of hiis home.
Taking what supplies he could scrouge, and his secret cache of potions that he was developing to mimic magical powers from legends and sets out to get his mother back. The sword One For All sheathed at his hip, where he can actually draw the long weapon. He trains with it during the evenings. Curiously, he begins hearing voices that instruct him, and feels a strange power building inside of him.The vestiges of the previous bearers work with the young boy to improve his chances.
I feel that he will visit the other members of Class 1-A along the way, so a quick run down of the characters under the readmore
