One idea could be that Harry Potter and his friends discover a magical tennis court at Hogwarts. The spells in the wizarding world could be used to enhance the tennis skills in a really unique way. For example, a charm could make the tennis ball move like a snitch, adding an element of magic to the game.
A possible plot could involve a tournament where both Harry Potter characters and Prince of Tennis characters participate. The wizarding world could provide magical obstacles on the tennis court, like moving hoops that the ball has to pass through. And the Prince of Tennis players could introduce their unique techniques to the wizarding world, causing a stir among the wizards who are used to more traditional magical sports.
I think Harry Potter himself would be very interesting. Imagining him learning tennis skills from the Prince of Tennis characters and using his wizarding abilities to add a new twist to the game. He could use his Patronus to guide the tennis ball or something like that.
In some fanfictions, the Hogwarts houses could be related to the different tennis teams in Prince of Tennis. For instance, Gryffindor could be paired with the Seigaku team as both value courage and determination. The characters' magical abilities might also come into play during tennis matches, like using spells to enhance their speed or accuracy on the court. It creates a whole new dynamic where the two different universes' elements blend together.
Well, perhaps the magical creatures from the Harry Potter universe end up in the '300' setting. Dragons could be used by the Spartans to fight their enemies. The centaurs could also join the battle, lending their archery skills. And Harry could learn some combat skills from the Spartans, which he could then use back in the wizarding world to fight Voldemort's Death Eaters.
Perhaps the Prince of Tennis characters get transported to the Naruto universe. They could be mistaken for some new kind of ninja at first. Then, they start to teach the ninja kids tennis, which becomes a new trend in the Hidden Leaf Village. The contrast between the two worlds' cultures and the characters' abilities would make for an exciting story. For example, Ryoma's cool and collected demeanor might be a contrast to Naruto's more boisterous personality.
One idea could be that Carly and her friends discover a magical portal in their apartment that leads to Hogwarts. They then have to navigate the wizarding world, with Carly being fascinated by all the magic. Meanwhile, Harry Potter and his friends are intrigued by the strange Muggle - like devices the iCarly gang has. They end up teaming up to solve a mystery that involves both the wizarding and Muggle worlds.
Well, perhaps the Death Note from the Bleach universe gets mixed up with the magic in Harry Potter's world. A powerful wizard could get hold of it and start using it in ways that disrupt the balance of life and death in both worlds. Harry and his friends would have to figure out how to stop this misuse, with the help of some Bleach characters who understand the true nature of the Death Note.
One idea could be that Eragon, while on one of his adventures in Alagaësia, accidentally creates a portal that leads him to Hogwarts. There, he meets Harry Potter and they quickly realize they have a lot in common. They could team up to fight against a dark force that somehow threatens both their worlds, like a new type of dark magic that combines the magic of Alagaësia and the dark arts from the wizarding world.
In a more comical plot, Hagrid could accidentally stumble upon a small Transformer while in the Forbidden Forest. Thinking it's a strange magical creature, he brings it back to Hogwarts. The Transformer, being a bit lost in this new magical world, causes all sorts of chaos in the castle. Harry and his friends have to figure out how to get the Transformer back to where it belongs while also dealing with the daily challenges at Hogwarts.
One possible plot could be that the Duel Monsters from Yu - Gi - Oh! are introduced to the wizarding world of Harry Potter. Wizards start using them in duels instead of their traditional spells. Harry and his friends have to learn the new rules of these duels and face off against a dark wizard who has mastered the most powerful Duel Monsters.
Well, perhaps Spyro discovers a magical portal in his world that leads to Hogwarts. Once there, he becomes a sort of mascot for Gryffindor because of his brave nature. The students try to learn from Spyro how to better communicate with magical beasts, as he has a special connection with them. Meanwhile, Voldemort or one of his followers might see Spyro as a threat and try to capture him. Harry and his friends then have to protect Spyro and in the process, learn more about Spyro's world and its magic.