A fanfiction could involve Harry Potter being transported into the world of Snow White. He might mistake the Queen for a dark wizard and try to stop her from harming Snow White. Along the way, he could use his magic skills from Hogwarts to outwit the Queen and protect Snow White, while also learning about the different kind of magic in this new world.
Perhaps there could be a fanfiction where the magic of the Harry Potter universe somehow gets mixed with the story of Snow White. The Queen could get hold of a Hogwarts spellbook and try to use the spells to make herself the most powerful in the land. Snow White, with the help of some friendly magical creatures from the Harry Potter world, has to fight back against the Queen.
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.
In a fanfiction, Ron could find himself in the world of Snow White. He might initially be confused but then team up with Snow White. When facing the Queen, he could use his Weasley family's unique magical items. The Queen, seeing these new forms of magic, would be both intrigued and threatened. And Dumbledore could make an appearance as a wise guide, teaching Snow White about the power of love and how it can be a stronger magic than the Queen's dark arts.
Another possibility is a story where the Cullen family's vampire lore intersects with the magic system in Harry Potter. Maybe there's a dark magic that affects vampires in a unique way, and Alice Cullen has to seek help from the wizarding world to deal with it. Harry and his friends, with their knowledge of magic, could offer solutions that the Cullens hadn't considered before.
Maybe Harry Potter and his friends visit Stark Tower in the Marvel universe. They are amazed by all the advanced technology there. Iron Man could be intrigued by magic and try to figure out how it works, leading to some really interesting interactions between science and magic.
One idea could be a story where a character from the Harry Potter world, perhaps a student at Hogwarts, discovers they have a connection to the werewolves in Twilight. Maybe they are bitten during a magical accident and start to show traits of both a wizard - with their magic - and a Twilight - style werewolf. This could lead to all sorts of interesting plotlines as they try to hide their new identity from both the wizarding world and any vampires or werewolves in the Twilight realm.