Now if you decide to do it here is my idea for Harry's Quirk which he has along with gaining all his cannon abilities which you have my blessing to use along with the basic idea following forward.
For Harry's Quirk why not base it around the abilities of the Phoenix and The Basilisk as Harry does have the Tears of the Phoenix and Venom of the Basilisk running in his veins now? Harry can regenerate his body such as his limbs and bones but this regeneration takes time along with Harry now having Pyro Kinesis and flight without a broom by flying on wings of Fire! And Harry's eyes can only petrify people like in Harry Potter and The Chamber of Secrets and Harry can turn this ability his eyes now have on and off at will but the petrification only works when making eye contact with the guy he wants to petrify? That would be awesome! Not to overpowered but enough to give Harry an edge against his opponents!
However I have no idea on how to make this happen. Maybe if Harry gets beaten up severely and needs a blood transfusion with someone with a Quirk which happens to rapidly develop and awaken Harry's Quirk and he decides to go to U.A High to learn to control it? That would be cool.
Oh and this could just be me but if you decide to do it Harry should expand his mind and learn other mystical practices. Speaking of which you should take a look at real world systems of mysticism like Norse magic's such as Gandr, Seidr, Runes(the Elder Futhark, the Anglo-Saxon Futhorc, the Younger Futhark, Rök runes, Hälsinge runes, Medieval runes, and the Dalecarlian runes), and Western Hermiticism and Western Alchemy.
Also befitting the fact that Merlin was in Harry Potter cannon and a student at Hogwarts Harry should learn Celtic Magic, Druidry, and Celtic Ogham befitting the old mythos of Merlin being a Druid, and some really old paths of Sorcery like Ancient Egyptian magic's along with Shamanism and all the arts associated and traditionally in Shamanism and Anime versions of Shamanism with all that entails in that art of magic along with getting better with his Fortune Telling skills such as learning about and using the Tarot. I am talking about The Arcana.
The Major Arcana consists of 22 cards without suits: The Magician, The High Priestess, The Empress, The Emperor, The Hierophant, The Lovers, The Chariot, Strength, The Hermit, Wheel of Fortune, Justice, The Hanged Man, Death, Temperance, The Devil, The Tower, The Star, The Moon, The Sun, Judgment, The World, and The Fool. Cards from The Magician to The World are numbered in Roman numerals from I to XXI, while The Fool is the only unnumbered card, sometimes placed at the beginning of the deck as 0, or at the end as XXII.
The Minor Arcana consists of 56 cards, divided into four suits of 14 cards each; ten numbered cards and four court cards. The court cards are the King, Queen, Knight and Page/Jack, in each of the four tarot suits.
Now seeing as Harry is living and training in the far east he should learn East Asian magic's like Taoism, Onmyōdō, Chi Magic, and East Asian Alchemy along with awakening his chakra's by following Buddhist practices just to gain enlightenment superpowers.
For pairings of both the Harry Potter and My Hero Academia casts... So long as Harry is still a straight male and you keep the characters as much as themselves as possible including their cannon sexuality I have no problems with who you choose to pair up with who.