Well, perhaps there could be a situation where a character from True Blood, like Sookie Stackhouse, somehow ends up at Hogwarts. She has her telepathic abilities which could be really interesting in the wizarding world. The Hogwarts students might be both fascinated and a bit scared of her. And then, she could get involved in some sort of mystery that Harry Potter and his gang are also trying to solve. Maybe there's a dark magic that is affecting both the magical folk and the vampires, and they have to work together to stop it.
Perhaps in the crossover, the Prototype virus could somehow infect the magical creatures in the Harry Potter universe. This would lead to a crisis where normal spells and magic can't stop the infected creatures. Harry and his friends would have to seek help from Mercer, who understands the virus. Together, they could try to find a way to either cure the infected or contain the spread. Mercer might use his own knowledge of the virus and his powers to fight the infected creatures, while Harry uses his magic skills to protect Mercer from magical attacks.
One possible plot could be that the One Ring somehow finds its way into the wizarding world. Harry and his friends discover it and realize its dark power. Meanwhile, some Hobbits from Middle - earth are accidentally transported to Hogwarts. They team up with the Hogwarts students to keep the Ring away from the dark wizards who want to use it for evil, just like Sauron in their world.
A possible plot is that a powerful magical artifact in the wizarding world is actually a key to an ancient Stargate on Earth that has been long forgotten. Harry and his friends, along with the Stargate team, embark on a quest to find this Stargate. Along the way, they face challenges from dark wizards who also want the artifact for their own evil purposes and the Goa'uld who sense the power of the artifact. They must work together, using both magic and technology, to reach the Stargate and unlock its secrets.
Maybe there's a prophecy in both worlds that foretells of a combined force from the wizarding world and the ninja world being the only way to defeat an ultimate evil. So, Naruto and Harry are brought together. They face challenges in understanding each other's cultures and powers at first. But as they train together, they find ways to merge their skills. For instance, Naruto's rasengan could be enhanced with a magical spell to create a more powerful attack.
One idea could be that the vampires in True Blood are discovered by the wizarding world in Harry Potter. Wizards might be intrigued by the different nature of these vampires compared to their own magical creatures. For example, they could study how True Blood vampires react to magic spells.
A possible plot is that Voldemort discovers the existence of the demigods and tries to use their powers for his own evil purposes. He sends his Death Eaters to capture some demigods. Percy and his friends manage to escape and seek refuge at Hogwarts. At Hogwarts, they learn about magic in a whole new way while also teaching the wizards and witches about their own powers. The two groups then have to combine their skills, like the demigods' combat abilities and the wizards' spell - casting, to fight against Voldemort and his new - found allies from the world of Greek mythology.
One idea could be that Voldemort discovers a way to enter the DC universe and teams up with some of the Justice League's villains like Lex Luthor. The Hogwarts students and the remaining members of the Justice League have to join forces to stop them. The wizards can use their magic while the superheroes use their powers.
One possible plot could be that the Death Eaters in the Harry Potter world team up with the Akatsuki from Naruto. Harry and his friends, along with Naruto and his comrades, have to join forces to stop this powerful alliance. They could learn from each other's magic and jutsu. For example, Naruto could teach the wizards about chakra control, and the wizards could show Naruto some advanced spells for defense and offense.
The plot could involve a mystery surrounding this person with the blood of the four houses. They could be hunted by dark forces who want to use their special blood for evil purposes. At the same time, they are trying to uncover secrets about their heritage within the four houses.
One idea could be a story where the vampires in True Blood are exposed to the magic of the Harry Potter world. Maybe some of the True Blood vampires try to learn magic from Hogwarts students or teachers. Another idea is a plot where the werewolves in True Blood interact with the werewolves in Harry Potter, like Lupin meeting the True Blood werewolves and comparing their different experiences.