Perhaps Voldemort hears about Thor's powerful hammer and tries to steal it to gain more power. Thor then teams up with Harry and his friends to stop Voldemort. This would bring together the Norse mythology elements of Thor with the dark and dangerous world of the Death Eaters in Harry Potter.
One idea could be that Thor somehow ends up at Hogwarts. Maybe he's on a mission in the wizarding world and his Asgardian magic interacts in strange ways with the magic of Hogwarts. For example, his lightning powers could cause some chaos during a Potions class when it accidentally zaps a cauldron.
Another concept could be that Hogwarts students are taken on a field trip to Asgard. There, they learn about Asgardian magic and culture. Harry, being the curious and brave one, gets into some trouble while exploring Asgard. Thor has to help him out, and in the process, they exchange knowledge about their respective magics.
A great crossover idea might involve Harry sharing his knowledge of death and resurrection with Thor. Thor, having faced death and loss in his own world, could be intrigued. Maybe they could work together to bring back a fallen hero from either of their worlds. Also, the Elder Wand could have some sort of interaction with Thor's hammer. What if the power of the Elder Wand could enhance the abilities of Mjolnir for a short time?
A possible crossover idea is that a powerful villain from the Thor universe, say Loki, decides to use the chaos of the Naruto world to his advantage. Thor and Naruto then have to put their differences aside, as Thor comes from a world of gods and Naruto from a world of ninjas, and combine their strengths to defeat Loki and his new - found allies in the Naruto world.
Another possibility is that Voldemort somehow discovers the world of Narnia and tries to conquer it. This would force the Narnian characters to team up with the Order of the Phoenix. Harry Potter and his friends could learn about the talking animals and different forms of magic in Narnia, while the Narnian heroes would see the power of wizardry. It could lead to a great adventure full of battles and new friendships.
A cool Harry Potter OC Crossover Fanfiction idea could be to have an OC crossover with the world of Middle - earth. Imagine an OC from Hogwarts traveling to Middle - earth and using their magic to help in the battles against Sauron. They could introduce wands and spells to the hobbits and elves. Also, a crossover with the world of Fairy Tail would be interesting. The OC could join the Fairy Tail guild and participate in their quests while also showing off the unique magic of the Harry Potter universe. And a crossover with the world of Howl's Moving Castle could be fun too. The OC could interact with Howl and Sophie and learn about their form of magic which is different from Hogwarts magic.
One idea could be a magical exchange program. Wizards from Hogwarts and Dragon Riders from Alagaësia meet and learn from each other's magical systems. Harry Potter characters could be fascinated by the bond between Eragon and his dragon Saphira, while Eragon might be intrigued by the variety of magic at Hogwarts like Transfiguration and Potions.
One idea could be having Harry Potter characters attend an anime - style magic school in Japan. For example, Harry could be a transfer student to a school where they teach magic in a very different way compared to Hogwarts, with elements like anime - inspired uniforms and more focus on martial arts - based magic.
Perhaps there could be a magical object in 'Harry Potter' that is somehow related to the werewolves in 'The Howling'. For example, a spell book that has the secrets to controlling werewolf transformations. The characters from both worlds could be on a quest to find this book and in the process, they learn more about each other's worlds and how magic works differently in each.
A great idea for 'harry potter and friends crossover fanfiction' is a crossover with 'Narnia'. The magical world of Narnia and Hogwarts have some similarities. Harry and his friends could enter Narnia through a magical portal in Hogwarts. They could help the Pevensie children in their battles against the White Witch. Another option is a crossover with 'The Chronicles of Prydain'. Harry and his friends could journey to Prydain and encounter the magical creatures and challenges there. And a crossover with 'Fantastic Beasts' but in a different way, like the main characters from 'Harry Potter' meet Newt's ancestors and have adventures with them.
Maybe in this fanfiction, the Death Eaters are somehow connected to an evil ninja clan in the Naruto universe. Harry and Naruto then have to team up to fight against this combined threat. Naruto could teach Harry how to use chakra to enhance his physical abilities, and Harry could show Naruto some of the more subtle uses of magic, like invisibility spells that could be useful for stealth missions.
Maybe a Witcher potion - making concept could be introduced to the Hogwarts potions class. For example, a character from Harry Potter, like Hermione, could be intrigued by the complex and powerful Witcher potions. She might try to study them and combine them with her own magical knowledge. This could lead to new discoveries in both magical systems.