However, I think it could also be Todoroki. His dual - element quirk of ice and fire is very versatile. In the Demon Slayer world, he could use his ice to freeze demons in place and his fire to incinerate them. His control over his quirk has improved over time, and in a combined world, he could use it to great effect against both demons and any villains from the My Hero Academia side who might oppose him.
It could be All Might. His One For All quirk gives him superhuman strength, speed, and durability. In the Demon Slayer world, he would be able to use these powers to take on even the strongest demons with ease. His experience as a hero would also give him an edge in strategizing against the demons.
Probably a combination of Deku and Ken. Deku's One For All power gives him incredible strength and speed, and Ken's Shoryuken and other moves would add a whole new level of combat ability. Deku could use Ken's fighting style to better control his power, and Ken could benefit from Deku's raw power. They could take on any enemy in the combined universes with ease.
Probably a combination of All Might and Gol D. Roger. All Might has incredible strength and the symbol of peace status. Roger was the Pirate King with great leadership and combat skills. Together, they would be almost unbeatable, having the best of both the hero and pirate worlds in terms of power and influence.
Scorpion could also be a top - tier powerful character in this fanfiction combo. His hellfire abilities and his ninja skills are no joke. In a fight against the My Hero Academia characters, he could easily surprise them with his teleportation and his ability to throw his spear. However, someone like Endeavor with his powerful fire quirk could also put up a good fight against Scorpion.
Combining Doctor Strange and Shoto Todoroki could result in a very powerful character. Doctor Strange has his mastery over magic, with the ability to create portals, manipulate time, and use various spells. Shoto Todoroki has his ice and fire Quirks. The combination would mean that this new character could use ice and fire attacks while also being able to use magic to control the battlefield. They could freeze enemies and then shatter them with fire spells or use portals to surprise opponents.
It could be All For One from My Hero Academia if he managed to absorb the powers of some Halo characters. Imagine him having the strength and abilities of a Spartan on top of his already formidable power - stealing Quirk. He would be almost unstoppable.
Tanjiro and Deku would be a very interesting pairing. Tanjiro has his strong sense of justice and his skills as a Demon Slayer, while Deku is determined to become the greatest hero. They could learn a lot from each other, like Tanjiro teaching Deku about perseverance in the face of great odds, and Deku showing Tanjiro how to better analyze an opponent's abilities.
A character like Kaido with a quirk that made his already tough body even more indestructible would be a nightmare to face. On the other hand, Deku with the ability to use all the One Piece Haki types in addition to his multiple quirks would be an absolute powerhouse. He could use Armament Haki to strengthen his punches even more, Observation Haki to predict his opponents' moves better, and Conqueror's Haki to intimidate his foes. In a fanfiction world, the possibilities for creating the most powerful character are endless.
Sasuke with a quirk could also be extremely powerful. If he had a quirk like Shoto Todoroki's ice and fire ability on top of his Sharingan and Rinnegan, he would be a force to be reckoned with. He could use his ice to freeze opponents from a distance, then use his fire to create powerful explosions, all while using his Sharingan to predict their moves and his Rinnegan for more advanced techniques.
In a fanfiction setting, Bakugo might be the most powerful. His explosive quirk is already very destructive. At Hogwarts, he could learn some offensive magic spells and combine them with his quirk for some really powerful attacks. He's also highly competitive and would strive to be the best, which would drive him to master both his quirk and magic faster than others.
It's likely Goku. His ability to constantly reach new levels of power like Super Saiyan God and Ultra Instinct is far beyond what most My Hero Academia characters can achieve. Even the strongest in My Hero Academia, like All Might at his prime, would have a hard time against Goku. Goku can destroy planets, while the power in My Hero Academia is more focused on individual combat and saving cities rather than galactic - level threats.