One great 'My Hero Academia' crossover fanfiction is the one where they cross over with 'One Piece'. It combines the unique powers of the 'My Hero Academia' world with the adventurous and diverse world of 'One Piece'. The characters interact in really interesting ways, like Midoriya being amazed by Luffy's Devil Fruit powers and Luffy being intrigued by the concept of Quirks.
First, know both universes well. For a 'hero academia crossover fanfiction', you need to understand the rules of the Hero Academia world like how Quirks work, and also the other universe you're crossing it with. For example, if it's a crossover with 'Star Wars', understand the Force and lightsaber combat. Then, create a good plot that makes sense for characters from both worlds to be involved. Maybe a threat that only they can stop together.
One idea could be that Kakashi is somehow transported to the My Hero Academia world. He could become a teacher at UA, using his ninja skills and wisdom to train the students in new and unique ways. His Sharingan would be a very interesting power in the MHA universe.
A great crossover idea is with the Dragon Ball series. The powerful Saiyans could interact with the My Hero Academia characters. Goku could train the students at UA to reach new levels of power. Also, a crossover with the Attack on Titan world would be intense. The heroes would have to deal with Titans in a whole new way and use their quirks in that battle - filled environment.
One idea could be that Godzilla arrives in the My Hero Academia world during a major villain attack. The heroes initially see Godzilla as a threat but soon realize that he is targeting the same villains who are causing environmental destruction. His massive size and power could be a game - changer in the battle.
The Powerpuff Girls. Blossom, Bubbles and Buttercup in the world of My Hero Academia would be amazing. They could team up with the female heroes like Uraraka and Tsuyu. Their superpowers could combine with the quirks in some really unique battles against powerful villains.
One idea could be a crossover with the Naruto universe. The characters from My Hero Academia could interact with Naruto characters. For example, Deku might learn some new fighting techniques from Naruto. Another idea is a crossover with the Harry Potter world. The students at U.A. could visit Hogwarts and experience magic, while the Hogwarts students could learn about quirks.
In this crossover, the Avengers could learn a lot from the students at U.A. High. For example, Midoriya's determination might inspire Captain America. And the Avengers' high - tech gadgets could be a wonder to the My Hero Academia characters. Maybe Iron Man would be interested in improving Mei Hatsume's inventions.
There are quite a few great crossover fanfictions between Avengers and My Hero Academia. One that stands out involves the Avengers being transported to the My Hero Academia universe. In this story, they have to adapt to the world of quirks while also trying to find a way back home. It's really interesting to see how characters like Iron Man interact with the likes of All Might.
One great 'hero academia crossover fanfiction' I read was a crossover with 'Avatar: The Last Airbender'. It was really interesting to see how the characters from Hero Academia would interact with the bending powers and different cultures in the Avatar world. For example, Bakugo trying to learn firebending and getting frustrated with the spiritual aspects.