Maybe Ichigo is a student at Hogwarts. He could be in a different house than the typical ones. He'd bring his sense of justice from the Bleach universe into Hogwarts and get involved in solving mysteries that involve both magical and spiritual elements. Like, there could be a Dementor - like creature that is actually a Hollow in disguise, and Ichigo would be the only one to notice the difference.
One idea could be that Ichigo is somehow transported to the wizarding world. He might use his Soul Reaper powers in unique ways against dark wizards. For example, his Zangetsu could be seen as a sort of magical weapon that even wizards are in awe of.
One idea could be that Ichigo has to go on a special mission with a group of characters and during the journey, they all discover new powers within themselves. For example, Ichigo could learn a new form of his zanpakuto from an unexpected ally.
Maybe in the fanfiction, Ichigo and Naruto meet during a tournament held in a world that combines the elements of their respective universes. They start as rivals but soon realize they have more in common and end up fighting together against those who want to disrupt the tournament for their own evil purposes. This could involve them combining their unique powers like Ichigo's Getsuga Tensho and Naruto's Rasengan in creative ways.
Maybe in a modern AU (alternate universe), Ichigo is a regular high school student and Lisa is a new transfer student. They meet in the library where Ichigo is studying for an exam and Lisa is looking for some rare books. Their initial encounter leads to a series of events that bring them closer together, like working on a school project.
One idea could be a story where Ichigo and Shirosaki are forced to team up against a new, powerful enemy that threatens both their existences. They have to learn to trust each other despite their differences.
Maybe Alucard is a time - traveler who accidentally ends up in the era of Harry Potter. He initially has his own agenda but gets involved in the fight against Voldemort. His unique powers, which are very different from the magic in the wizarding world, make him both an asset and a mystery. Harry and his friends are both fascinated and a bit afraid of him. Alucard could also introduce a new type of magic or power that the wizards have never seen before, which could change the balance of power in the war against the Dark Lord.
One idea could be that a character from Harry Potter, like Hermione, accidentally gets transported to Storybrooke. She could use her magic knowledge to help the residents solve a mystery that has been plaguing the town. Maybe it's a spell that has gone wrong and is causing strange things to happen, and her logical thinking and spell - casting skills would be very useful.
One idea could be that Slenderman somehow enters the wizarding world of Harry Potter. Maybe he's drawn by the powerful magic there. He could start causing strange disappearances in the Forbidden Forest, and Harry and his friends have to figure out how to stop this new, otherworldly threat using both their magic skills and their knowledge of the wizarding world's secrets.
One plot idea could be that a Kitsune disguises itself as a student at Hogwarts to learn more about human magic. It could get involved in Harry's adventures and use its Kitsune abilities, like shape - shifting or illusion - casting, to help or sometimes cause trouble. Another idea is that a Kitsune is a guardian spirit of an ancient magical place near Hogwarts, and Harry discovers it during one of his explorations. The Kitsune then becomes an ally or a guide for Harry in uncovering secrets about the wizarding world.
One idea could be that Shikamaru, with his strategic mind, gets transported to the wizarding world of Harry Potter. He could use his intelligence to help Harry and his friends in their battles against Voldemort. For example, he might come up with unique battle plans that the wizards hadn't considered before, like using the environment in Hogwarts in unexpected ways during duels.
Well, a plot could be Loki disguising himself as a Hogwarts student. He gets involved in the house rivalries and eventually meets Harry. They start off as adversaries but then realize they both have a common goal of protecting the wizarding world from a hidden danger. For example, an ancient evil that is about to be awakened. Loki could use his Asgardian knowledge and Harry his wizard skills to defeat it. Also, Loki could be interested in the magic of the wizarding world and want to learn more, leading to some humorous and interesting interactions with the Hogwarts professors.