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.
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 fanfiction could revolve around a powerful demon who has been trapped in the wizarding world for centuries. Harry Potter and his friends come across this demon and find out that it holds the key to defeating a new, even greater dark force that is threatening both the wizarding and muggle worlds.
Perhaps a fanfiction where a young witch or wizard from Hogwarts has a natural affinity for Storm's powers. They could be an outcast at Hogwarts because of their different abilities. But when a great danger looms over both the magical world and the world of Storm, they are the key to bringing both sides together. Their unique combination of magic and Storm powers helps to save the day in a very unexpected way.
In 'Harry Potter x Pokemon fanfiction', an interesting concept could be the magical creatures in the Harry Potter universe interacting with Pokemon. For instance, the Hippogriffs could befriend Flying - type Pokemon, and the Centaurs could have some sort of relationship with Grass - type Pokemon. Also, the spells in Harry Potter could have an impact on Pokemon. Maybe the Stupefy spell could temporarily paralyze a Pokemon, and the Protego spell could protect against a Pokemon's attack. This would create a whole new dynamic between the two universes, and characters from both worlds would have to adapt to these new interactions.