Another interesting concept is to have the vampires from Hellsing be affected by the magic in the Harry Potter universe. For example, they could find that some spells either enhance or nullify their powers. This could lead to a complex power struggle and new alliances. Perhaps Hermione would be intrigued by the different nature of the Hellsing vampires compared to the magical creatures she's used to and try to study them, which could cause all sorts of interesting interactions.
One idea could be that the wizards in the Harry Potter world discover the existence of the Hellsing Organization. Maybe Harry and his friends end up teaming up with Alucard to fight a common dark magic threat that is beyond what the wizarding world alone can handle.
One idea could be to have Alucard from Hellsing meet the vampires in Twilight. Maybe Alucard would be amused by their 'tame' nature compared to his own blood - thirsty self. He could show them what a real vampire is capable of in a fight.
Another possibility is a crossover with 'Tokyo Ghoul'. The ghouls' unique abilities and the vampire - hunter theme of 'Hellsing' could make for a really intense and dark fanfiction. The struggle between the two species in a shared world would be full of drama and action.
One idea could be a crossover with 'Berserk'. The dark and gritty worlds of both could blend well. The vampires from Hellsing could interact with the Apostles from Berserk, leading to epic battles. Another could be a crossover with 'Fullmetal Alchemist'. Imagine the alchemists trying to understand the nature of the vampires in Hellsing. Maybe they'd try to find a way to use alchemy on them.
One idea could be a battle between the forces of Hellsing and some unique threat from the Alexandee universe. Maybe the Hellsing crew has to deal with a powerful magic user from Alexandee who is causing chaos in their 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.