A combination of Shoto Todoroki and Ace could be very powerful. Todoroki has his ice and fire powers, and Ace had his fire - based Devil Fruit powers. The two fire - based powers together could create an extremely intense and destructive force, and the addition of Todoroki's ice could also be used for defense and strategic attacks. They would be a force to be reckoned with in any crossover scenario.
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.
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.
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.
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.
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.
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.
It could be All For One fused with Thanos. All For One has the ability to steal and accumulate quirks, and Thanos is extremely powerful with his Infinity Stones. Combined, they would have a vast array of powers and a thirst for control that would make them nearly unstoppable. Their knowledge of different universes' powers and resources would give them an edge over any opposition.
It's hard to say for sure. Naruto in his final form with all his chakra and Sage Mode abilities is extremely powerful. He could potentially overpower many of the My Hero Academia characters. But All Might at his peak was also incredibly strong. His punches could cause massive destruction. However, if we consider characters like Kaguya from Naruto, with her god - like powers, she would likely be one of the most powerful in a crossover. She has abilities like creating dimensions and is almost unbeatable in the Naruto universe.
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.