One idea could be to have the characters from Harry Potter world attend the Vongola Academy in the KHR universe. They could learn about the different flames and how to use them while also sharing their magical knowledge. For example, Hermione would be really interested in the scientific aspects of the Dying Will Flames and might try to find a way to combine it with her magic.
Maybe a crossover where the Death Eaters team up with the Estraneo Famiglia for some dark purpose. This would force the protagonists from both universes, like Harry and Tsuna, to work together. They would have to overcome their differences in fighting styles and magic systems. Harry's use of spells and Tsuna's use of his Hyper Dying Will Mode would create really cool battle scenes.
A fanfiction could explore what would happen if the Sorting Hat was placed in the KHR world. Which family would it sort the characters into? Would it recognize the qualities of the Vongola Guardians? It could be really interesting to see how the Hat's magic interacts with the unique culture and abilities in the KHR universe.
One interesting Harry Potter KHR fanfiction could be one where the magic systems of both worlds are combined. For example, characters from KHR might learn some spells from the Harry Potter universe and vice versa. This creates unique and exciting plotlines as they navigate the differences and similarities of the two magical realms.
One idea could be a time - travel plot where Tsuna is sent back in time and has to rely on Reborn's teachings from the start again, but with the knowledge he already has. This would lead to some really different interactions and a chance to change certain events in a more strategic way.
One interesting idea could be the combination of magic systems. In 'Harry Potter', there's wand - based magic, while in 'Bleach' there are spiritual powers. Maybe in the fanfiction, characters from both universes learn to use the other's power source. For example, Harry Potter could learn to use reiatsu (spiritual pressure from 'Bleach') to enhance his spells, or Ichigo from 'Bleach' could learn how to use a wand to focus his spiritual energy in a new way.
One idea could be a story where a character from Harry Potter accidentally travels to the Hearime world and has to adapt to its unique magic system. They could learn new spells and make friends with Hearime characters while trying to find a way back to their own world.
One idea could be a crossover where Noctis visits Hogwarts. He could be mistaken for a new kind of wizard with his unique powers. Maybe he gets sorted into a house, like Gryffindor for his bravery, and then he has to learn about magic in the wizarding world while also sharing his own skills with the students.
One idea could be that Hogwarts students visit Pandora. They could learn about the unique magic of the Na'vi, like their bond with the animals. Maybe a Hogwarts professor is intrigued by the Eywa and wants to study it. Another could be a character from Harry Potter getting lost in Pandora and having to adapt to the Na'vi way of life while also using their own magical skills to survive.
One idea could be that Azula, with her firebending skills and strategic mind, is somehow transported to the Harry Potter universe. She could be sorted into Slytherin and form an unlikely alliance with Draco Malfoy. They might plot together to gain power within Hogwarts, using Azula's manipulation tactics.
Another interesting concept is a crossover where the magic system of Harry Potter and the power dynamics in Overlord combine. For instance, Voldemort could make a deal with one of the antagonists in Overlord to gain more power. This would lead to a complex web of alliances and betrayals as the heroes from both worlds try to stop them. The Hogwarts students might be trained in new combat styles inspired by Overlord's battles.
One idea could be that McGee discovers he has a hidden magical ability and is sent to Hogwarts. There he meets Harry Potter and they become friends. McGee could use his Muggle knowledge in unexpected ways in the wizarding world.
A possible fanfiction could involve a time - travel scenario. Tony accidentally travels back in time to the era of Harry Potter. He realizes that he can use his knowledge to prevent some of the tragedies in the wizarding world. He partners with Harry and uses his Iron Man suit's capabilities along with magic to change the course of events.