One common crossover idea is a scenario where the Titans from Attack on Titan are considered as 'villains' in the My Hero Academia universe, and the heroes have to come up with strategies to defeat them. It could also be interesting to see how the Quirks in My Hero Academia would be perceived in the world of Attack on Titan. Maybe some characters would be seen as 'demons' because of their unique powers. And then there's the idea of a character from Attack on Titan, like Eren, somehow getting a Quirk in the My Hero Academia world and how that would change his character arc.
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 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.
A cool My Hero Academia crossover fanfiction idea would be with the Avengers. The heroes from UA could team up with the Avengers to fight a common enemy. For example, a powerful villain who can absorb quirks and powers. It would be interesting to see how characters like All Might and Captain America interact, and how their leadership styles differ.
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.
One great place to find My Hero Academia and Attack on Titan fanfiction is Archive of Our Own (AO3). It has a vast collection sorted by various tags like character pairings, genres, and ratings. Another option is Fanfiction.net. There are also some Reddit communities dedicated to fanfiction where users share and discuss their favorite works.
One popular idea is the relationship between Midoriya and Bakugo. Fans like to explore how their complex dynamic could develop into something more in a slash context. Another is Todoroki and Deku, imagining their friendship evolving into a romantic relationship. And also Kirishima and Bakugo, as their strong bond can be seen in different ways in slash fanfictions.
One interesting idea could be a crossover with another popular superhero universe, like Marvel or DC. This could lead to epic battles between different types of heroes. Another idea might be a crossover within the My Hero Academia world itself, for example, different classes or generations of heroes teaming up in unexpected ways.
A great fanfiction idea would be to have some of the My Hero Academia students train with Batman. He could teach them his stealth and combat techniques. This would be especially interesting for characters like Todoroki. Batman could also help them understand the importance of psychological warfare against villains. In addition, there could be a plot where a new villain emerges in the My Hero Academia world, and Batman has to use his knowledge of Gotham's villains to help the heroes figure out how to defeat this new threat.
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.
Well, a really common trope is the 'Team - Up' one. You'll often see My Hero Academia characters teaming up with characters from other shows or universes. It creates interesting dynamics as they combine their different powers and personalities. Then there's the 'Mystery Visitor' trope, where a character from another world suddenly shows up in the My Hero Academia world and everyone has to figure out who they are and what they want. And of course, the 'Battle Royale' trope, where characters from different worlds are pitted against each other in a big fight.