Perhaps the characters from Fast and Furious accidentally stumble into the wizarding world. They then get involved in a race against time, literally, as they help Harry Potter and his friends stop a time - related dark spell. The cars could be enchanted to fly or perform other magical feats.
One idea could be that the wizards in the Harry Potter world need to learn some fast - paced driving skills from the Fast and Furious crew to escape a new dark magic threat that is spreading quickly. They could use magic to enhance the cars' capabilities.
One interesting plot idea is having the characters from Fast and Furious enter the Triwizard Tournament in the Harry Potter world. Imagine Dom Toretto competing in a magical challenge on a broomstick. Another is Harry Potter characters using magic to help the Fast and Furious gang in a high - stakes car chase against some dark magic - using villains.
One way to combine them could be to have the characters from Fast and Furious enter the wizarding world of Harry Potter. For example, Dominic Toretto could drive his car into Hogwarts, causing a stir among the students and teachers. Maybe his car has some sort of magical ability that gets noticed by the wizards.
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.
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 a crossover where Kakashi is summoned to the Harry Potter world by a dark magic spell gone wrong. He then uses his ninja skills and Sharingan to navigate the wizarding world, perhaps even teaching some Defense Against the Dark Arts classes with his unique combat knowledge.
Maybe in a fanfiction, Harry Potter and his friends accidentally stumble upon a hidden Cybertronian relic. This relic has some sort of power that attracts Megatron. Harry and his friends then have to defend the relic from Megatron while also trying to figure out what it does and how to use it to their advantage in the fight against Voldemort.
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 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.
Another concept could be a crossover where the magic in the Harry Potter universe has an unexpected effect on Android 18. For example, it could start to interact with her android systems in strange ways, giving her new powers or abilities that are a combination of magic and her android technology. This would lead to a whole new set of adventures as she tries to figure out what's happening to her.
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.