One idea could be a crossover where Sasuke is somehow transported to the Harry Potter universe. He could attend Hogwarts and his ninja skills might be mistaken for advanced magic. Maybe he gets sorted into Slytherin because of his ambitious nature.
A fanfiction could revolve around a magical artifact in the Harry Potter world that has the power to bring fictional characters to life, and it brings Sasuke. He then teams up with Harry Potter to fight against a common enemy, like a dark wizard who is also after the power of the Sharingan.
One idea could be a crossover where a character with Uzumaki and Uchiha blood somehow ends up in the Harry Potter world. Maybe they have unique magical abilities that combine the Sharingan's powers with the Uzumaki's chakra control, and they attend Hogwarts. They could face off against dark wizards using their unique skills.
Another concept could be a story where the magic in the Harry Potter world and Itachi's ninjutsu are somehow combined. Imagine Itachi using his fire - style jutsu along with a spell from the Harry Potter world to create a unique and powerful attack. This could be used against a common enemy that threatens both the ninja world and the wizarding world.
A different fanfiction could involve a magical experiment gone wrong that transports Harry Potter to the Naruto world where he meets Obito. Harry, with his knowledge of magic, and Obito, with his ninja skills, have to find a way back to their respective worlds. Along the way, they learn from each other's cultures and abilities. Harry could teach Obito about some basic spells, and Obito could show Harry some ninja techniques like how to move stealthily or use a kunai effectively.
Perhaps a fanfiction where Sasuke discovers a Portkey in his world that leads him to Hogwarts. He has to adapt to the new magical environment and forms an unlikely friendship with Harry Potter. Together, they explore the secrets of both their worlds and use their combined skills to face challenges.
Another concept could be that Harry Potter discovers he has a connection to Magneto's mutant powers. This might happen during a strange encounter with a dark magic object that unlocks a latent power within him. He then has to learn how to control these new abilities while also dealing with the threats in his own wizarding world, and he might seek out Magneto for guidance in a secret meeting across the boundaries of their two worlds.
One idea could be that Hawkgirl somehow gets transported to the Harry Potter universe. Maybe she arrives during a Quidditch match and her flying skills astound everyone. She could then join forces with Harry and his friends to fight against the dark forces in Hogwarts, using her Nth metal mace and flight abilities in unique ways.
One idea could be a crossover where Harry Potter accidentally ends up in the world of Ranma 1/2. Maybe he's on a magical experiment gone wrong and lands in Nerima. He could use his magic to help Ranma deal with his curse, like creating a spell to keep him from changing into a girl when splashed with cold water.
One idea could be a crossover where Harry Potter accidentally travels to the One Piece world and meets Whitebeard. He could learn about their different forms of magic and power, and together they face a new threat that combines the dark magic from his world and the powerful pirates in the One Piece world.
One idea could be that Fluttershy somehow gets transported into the Harry Potter world. Maybe she arrives at Hogwarts as a new kind of magical creature that the students have never seen before. Her gentle nature might initially cause confusion among the more rowdy students, but she could gradually make friends with the likes of Luna Lovegood, who would be intrigued by her uniqueness.
One idea could be that Gandalf somehow gets transported to the Harry Potter universe. He could become a mentor to Harry, using his vast magical knowledge and wisdom from Middle - earth. Maybe he helps Harry understand some ancient magic that even Dumbledore wasn't fully aware of.