Genos from One Punch Man in Boku no Hero Academia. He's always seeking to become stronger. In Boku no Hero Academia, he could befriend characters like Bakugo who also have a strong drive for power. He could learn different ways of training and power - up from the Boku no Hero Academia universe.
Saitama in the world of Boku no Hero Academia would be really interesting. He's so overpowered compared to the characters in Boku no Hero Academia. His simple and straightforward personality would contrast with the more complex and emotional characters there.
Hisoka would be really interesting. His unpredictable and somewhat psychotic nature would create a lot of chaos in the My Hero Academia world. The heroes would be really challenged to deal with his nen abilities which are so different from quirks.
Maybe a story where the heroes from Boku no Hero Academia are faced with a villain so powerful that they have to seek the help of Saitama. It would be interesting to see how they interact with his nonchalant attitude towards heroics.
All Might and King would be an unexpected but interesting pairing. All Might is the symbol of peace in the MHA world, a paragon of heroism. King, who is known for being a false hero in One Punch Man (even though he has some hidden strengths), could have a really interesting dynamic with All Might. All Might might try to inspire King to be a true hero, while King could teach All Might some of his unique survival skills in a world full of powerful villains.
Reincarnating as Midoriya but with some changes in his quirk manifestation could be fascinating. For instance, what if instead of getting One for All in the normal way, the reincarnated Midoriya discovers a different way to unlock and use the quirk? This could lead to a whole new set of adventures and challenges, and also a different dynamic with the other characters.
Saitama would win. Saitama's whole shtick is that he can defeat any opponent with one punch. All For One is powerful in the My Hero Academia universe, but Saitama's strength is on a completely different level. His power is so absurd that it's hard to imagine any character from My Hero Academia being able to withstand his punch.
Midoriya and Naruto would be an interesting pairing. Midoriya's determination to become a great hero is similar to Naruto's drive to become Hokage. They could learn a lot from each other. Another would be Bakugo and Vegeta. Both are hot - headed and extremely powerful. They might start off as rivals but could end up respecting each other. And then there's Uraraka and Nami. Nami's skills with money and navigation could be useful to Uraraka, while Uraraka's quirk could be handy in combat situations for Nami.
Harry Potter himself would be an interesting character to crossover into the One Punch Man universe. He would be completely out of his element in a world full of superheroes and monsters. His magic skills would be a new and strange power there. He could team up with Saitama and learn about a different kind of heroism. Also, his experiences with fighting Voldemort could give him unique insights when dealing with the villains in One Punch Man's world.
I think Jack Sparrow from 'Pirates of the Caribbean' would be really cool to cross over with Hercules. Jack's wily and unpredictable nature would contrast with Hercules' more straightforward heroism. They could be on a quest for a magical artifact that has the power to either save or destroy the world. It would be full of adventure, humor, and unexpected twists.
I think Gandalf would be a very interesting character to cross over. He has so much wisdom and power in LOTR. In the Harry Potter world, he could share his knowledge of different magics and perhaps help Dumbledore in his fight against Voldemort. He could also introduce the concept of the greater good in a more profound way to the young wizards and witches.
Definitely Doctor Strange. He already has a deep understanding of magic in the Avengers universe. In the Harry Potter world, he could exchange knowledge with the wizards, learn new spells, and maybe even help improve the defenses of Hogwarts. He could also have some really cool battles against the dark wizards using his unique combination of magic and martial arts.