One possible plot could be that Mulan is magically transported to the wizarding world of Harry Potter. She could be mistaken for a new student at Hogwarts and placed in Gryffindor due to her courage. Then, she uses her martial arts skills to help the students in Defense Against the Dark Arts class. Maybe she teams up with Harry, Hermione, and Ron to face a new dark threat that is a combination of the magic in the wizarding world and some ancient Chinese magic that has somehow seeped into Hogwarts.
Perhaps Mulan is sent to the wizarding world on a mission from her own land. She arrives at Hogwarts disguised as a male student (like she did in her own story). She quickly catches the attention of the Golden Trio. Harry might notice something different about her and as they get to know each other, they discover that there are dark forces in both their worlds that seem to be connected. Mulan could teach the Hogwarts students some of her military strategies, and they in turn could help her understand magic better so they can all fight against the common threat.
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.
The Stargate could be a portal between different magical realms in the Harry Potter universe. For example, it could lead to a place where ancient and more powerful magic exists. Wizards from different schools around the world could team up with Stargate explorers to uncover the secrets of this new place. There might be a new threat emerging from this realm that requires both magic and Stargate technology to defeat. This would bring together characters like Harry Potter and Jack O'Neill to lead the charge against the common enemy.
Well, perhaps there's a rift in the multiverse that connects Central City to Hogwarts. The Flash, while chasing a villain, gets sucked into the wizarding world. Harry Potter and his friends initially suspect him as a threat but then realize his good intentions. They then combine their powers - the Flash's speed and the magic of the wizards - to fight a common enemy. For example, the Flash could use his speed to distribute protective spells all over Hogwarts really quickly, while Harry and his friends focus on the offensive magic against the bad guys.
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.