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 that the Hetalia characters suddenly find themselves in the BNHA universe and have to adapt to having quirks. For example, Italy might be really confused at first but then find that his ability to make good pasta could be turned into a quirk related to food creation or manipulation. Another could be that the BNHA characters visit the Hetalia world and are shocked by the different cultures and historical settings. They could learn about different forms of power and leadership from the Hetalia characters.
Maybe the villains from BNHA team up with some of the evil forces in the Overwatch world. Then the combined good guys, including both BNHA heroes and Overwatch agents, have to figure out how to defeat this new, dangerous alliance. It could lead to some really cool battles and character interactions, like All Might facing off against Doomfist.
One idea could be exploring their training days before they became so well - known. Maybe focus on how they overcame their individual weaknesses. For example, Mirio could be shown struggling with a new type of quirk - based obstacle and how he found a creative way to get past it.
Well, you could start with the costumes. In a genderbend fanfic, the female versions of male heroes might have more stylish or practical costumes. For instance, All Might as a woman could have a more form - fitting suit that still represents her power. You could also play with the backstories. Maybe the female - genderbent Deku had to face different kinds of discrimination in a male - dominated hero world. And of course, there could be new romances or different interpretations of existing friendships.
Another interesting concept is a chatroom that exists secretly among the villains. They plot their next moves, argue about the best ways to take on the heroes, and maybe even have some internal power struggles. It would be exciting to see their different personalities clash in this virtual space.
One idea could be a fanfic where Nezu takes on a mentorship role for a young and struggling hero student. He could use his intelligence and experience to guide the student through difficult challenges.
The most interesting part is seeing the different power systems interact. Nen from HxH and Quirks from BNHA are so different. For example, how would Gon's Jajanken deal with a villain's powerful Quirk? It creates a lot of unique battle scenarios.
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.
One idea could be that the villains release a powerful virus that turns most of the population into mindless zombies. The heroes have to not only fight the villains but also deal with the zombie horde. Another could be an apocalyptic event that wipes out most of the world's quirks, and the remaining heroes and villains must adapt to this new powerless world.
Perhaps the villains get hold of the truth serum and use it on a captured hero. This hero reveals some crucial information about the hero society's security measures. It then becomes a race against time for the other heroes to either rescue the captured hero or change the security systems before the villains can act on the information.