One great 'Haikyuu! Bnha Crossover Fanfic' is 'Volleyball Heroes'. It combines the unique powers from Bnha with the exciting volleyball matches of Haikyuu. The characters' interactions are really interesting, like when Midoriya tries to learn volleyball techniques from Hinata. It's a fresh take on both universes.
First, you need to be familiar with both universes. Know the characters, their personalities, and the rules of each world. For example, in Haikyuu, understand the volleyball strategies and the different teams. In Bnha, know the quirks and how they work. Then, think of an interesting concept for the crossover. Maybe it could be about the Bnha characters joining a volleyball tournament in the Haikyuu world.
One idea could be that the characters from Bnha are transported to the Warrior Cats world. For example, Deku could find himself in a Clan and have to learn the ways of the cats to survive. He might use his quirk in unique ways to help the Clan, like using One For All to jump really high to catch prey or to defend against other Clans.
In 'bnha kny crossover fanfiction', a prevalent theme is the clash of cultures. The very different worlds of My Hero Academia, with its modern superhero society, and Demon Slayer, which is set in a more historical - fantasy Japan, create interesting scenarios. There's also the theme of adaptation. Characters from one universe have to adapt to the rules and norms of the other. For instance, heroes having to deal with the existence of demons, or demon slayers trying to understand the concept of quirks. Additionally, friendship and cooperation across the two universes is often explored as a theme.
One idea could be a crossover with Harry Potter. Imagine Deku attending Hogwarts and learning magic while also trying to control his One For All quirk. He could make friends with Harry and they could share their experiences of being the 'chosen ones'.
One idea could be to have the characters from BNHA attend the Death Weapon Meister Academy in the Soul Eater universe. They could learn about soul resonance and how to handle weapons in a whole new way. For example, Deku could be paired with a unique weapon and struggle to master the new techniques.
Next, when writing the crossover, focus on how the characters would react to the new environment. If Bakugo were to enter the Soul Eater world, he'd probably be very frustrated at first with all the new rules. You should also make sure to balance the elements from both series. Don't over - emphasize one over the other. For example, give equal importance to the development of BNHA - style quirks and Soul Eater - style soul abilities.
A Tsunade crossover in BNHA fanfiction could involve her being from a different world but getting involved in the BNHA universe's affairs. She might end up in a fight against some of the villains. Given her short temper, she could be quite a force to be reckoned with. She could also form alliances with some of the heroes like All Might, sharing her knowledge of combat and medicine.
Tsunade could interact with Midoriya Izuku. Izuku is always eager to learn new things and improve his skills. Tsunade could see his determination and potential, and she might teach him some of her combat and medical knowledge. Maybe she would help him control his quirk better or give him some physical training tips.
One popular trope is the 'transfer student' where a character from another series enrolls in U.A. High. For example, a character from Naruto might show up with their unique abilities, creating interesting interactions with the BNHA characters. Another is the 'power swap' trope, where BNHA characters get the powers of characters from a different universe and have to adapt. And the 'alternate universe' trope is also common, like a world where heroes and villains have different alignments or the BNHA world has a different social structure due to the influence of another fictional world.