One idea could be that Cloud somehow gets transported to the wizarding world of Harry Potter. Maybe he arrives during the Triwizard Tournament and is mistaken for a new contestant. His combat skills from his own world would make him a formidable competitor, and he'd have to learn the rules of magic quickly.
Perhaps there is a prophecy in the Harry Potter world that speaks of a hero from another realm, which turns out to be Cloud. He teams up with Harry Potter to fight against Voldemort, who has somehow gained knowledge of Cloud's world and is trying to use its power for his own evil purposes. Cloud's Buster Sword and his limit breaks could be really interesting elements to incorporate into the magical battles in the wizarding world.
The relationship develops gradually. Cloud, being from a different world, has to adapt to the wizarding world to raise Harry. Initially, he may be a bit distant, but as he witnesses Harry's struggles against the forces of the Dark Arts, he becomes more involved. He starts to share his own experiences of fighting against powerful enemies in his world with Harry. This sharing creates a deeper connection between them. Harry begins to look up to Cloud not just as a protector but also as a source of wisdom. And Cloud starts to care deeply for Harry, seeing in him a chance to do something good in this new and strange world he has found himself in.
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.
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.