Maybe the Ministry of Magic discovers a portal to the Monster Hunter world. Concerned about potential threats, they send Harry Potter and a team of wizards to investigate. There, they team up with Monster Hunters. Harry learns about their weapons and tactics, and they all work together to stop a powerful monster that is trying to cross over into the wizarding world. It would be a story full of action, with the two different groups learning from each other.
A possible plot is that a powerful dark wizard in the Harry Potter world summons a horde of Monster Hunter monsters to cause chaos. Harry, Hermione, and Ron must figure out how to defeat these strange and powerful creatures. Along the way, they meet a Monster Hunter who has somehow ended up in the wizarding world. With his help, they learn about the monsters' weaknesses and use magic to adapt Monster Hunter strategies. This would lead to a great adventure where magic and Monster Hunter lore blend.
A cool plot might be that Voldemort somehow discovers the existence of Monster High and plans to use the unique powers of the monsters there for his own evil purposes. So, Dumbledore sends a group of Hogwarts students, including Harry, to Monster High to warn them and form an alliance. The Monster High students, led by Frankie Stein and Draculaura, join forces with the Hogwarts students. They then have to use a combination of magic and monster abilities to stop Voldemort's plan, which could involve some sort of dark spell that would not only affect the wizarding world but also Monster High if successful.
Obviously, Harry Potter himself could be a main character. He could be the one who bridges the gap between the two worlds, using his skills in magic and his courage to face the challenges of the Monster Hunter world. Hermione would also be a great main character. Her intelligence would help in understanding the new creatures and the Monster Hunter techniques. And perhaps a Monster Hunter from the Monster Hunter world could be a main character too, guiding Harry and his friends in that world.
Perhaps a group of wizards and witches form an alliance with Monster Hunters to stop a powerful dark magic - infused monster that is terrorizing both the wizarding world and the Monster Hunter's territory. They have to exchange knowledge, like the wizards teaching the Monster Hunters about potions for healing and the Monster Hunters showing the wizards how to use their unique weapons more effectively.
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.