Well, perhaps a spell in the Harry Potter universe goes horribly wrong and creates a rift that pulls in characters from Supernatural. Sam and Dean could then help the Hogwarts professors figure out how to close the rift while also dealing with the magical threats that they've never encountered before. Or, a powerful dark wizard could be making deals with demons, similar to what happens in Supernatural, and the combined forces of both worlds have to stop him.
One idea could be that the Winchester brothers from Supernatural end up in the wizarding world of Harry Potter. They could be on a hunt for a supernatural creature that has found a way to hide among the wizards. Another idea is for Harry Potter and his friends to encounter a case of demonic possession in Hogwarts, and they have to team up with the Supernatural crew to solve it.
A really cool concept would be if the angels and demons from Supernatural have some sort of connection to the magic in the Harry Potter world. Maybe the angels are trying to protect a certain magical artifact in Hogwarts because it has the power to either save or destroy both worlds. Harry and his friends, along with the Winchester brothers, get caught in the middle of this battle between the celestial and the magical. They have to learn about each other's worlds quickly in order to succeed in protecting the artifact and both of their universes.
One idea could be a crossover with the Lord of the Rings. Imagine Harry and his friends traveling to Middle - earth to help in the fight against Sauron. They could use their magic in new and unexpected ways in that different magical realm.
One idea could be a crossover with 'Doctor Who'. The Doctor and the Winchesters could team up to fight a time - traveling demon. Another could be a crossover with 'Buffy the Vampire Slayer', where Buffy and the boys from Supernatural join forces against a powerful vampire coven that has some sort of connection to the supernatural underworld in both universes. And a crossover with 'The X - Files' would also be cool, with Mulder and Scully investigating a case that turns out to be related to the monsters the Winchesters usually hunt.
One idea could be that Hogwarts students discover Pokemon in the Forbidden Forest. They could start training them and use their powers in magical duels. For example, Pikachu's electric attacks could be used against dark wizards. Another idea is that Pokemon could be used in the Triwizard Tournament tasks, adding a new and exciting element to the competition.
A more complex idea would be a plot where Voldemort somehow discovers the immortality of the vampires in the Twilight world. He tries to make an alliance with them to gain more power, but the vampires are hesitant as they don't want to get involved in the wizarding war. This would create a lot of political intrigue and power struggles.
One idea could be a wizard from the Harry Potter world accidentally traveling to Forks. Maybe they mistake the vampires in Twilight for some kind of dark magic creatures. They could then try to use their magic to either fight or understand them.
One idea could be a spell gone wrong in the wizarding world that somehow transports some Hogwarts students to Forks. They then have to deal with the vampire and werewolf situation there while also trying to find a way back home. Maybe Hermione could be intrigued by the science - like aspects of the Twilight world that seem so different from magic.
One idea could be that the characters from 'The Magicians' end up at Hogwarts for a magical exchange program. They'd have to adapt to the different magical rules and culture there. For example, Quentin might be fascinated by the concept of houses at Hogwarts, while Eliot could get into trouble for trying to introduce some of his more 'adult' magic tricks from Brakebills.
One idea could be that Chuck, with his knowledge of the supernatural, ends up at Hogwarts. Maybe he's there to investigate some strange magical occurrences that are linked to his world's demons or angels. He could form an unlikely friendship with Harry Potter, and together they figure out how to stop a dark force that's using both magic and the powers from Chuck's universe.
One idea could be that Voldemort discovers the Infinity Stones and tries to use their power to conquer both the wizarding world and the Muggle world. The Avengers, with the help of Harry Potter and his friends, must stop him. They could combine magical spells with superhero powers to create unique battle strategies.
One idea could be that Voldemort teams up with a powerful Marvel villain like Thanos. This would force the Avengers and the Harry Potter characters to join forces. The Avengers could bring their strength and high - tech weapons, while the Harry Potter characters use their magic. Another plot could be that a magical artifact from the Harry Potter world is lost in the Marvel universe and both groups have to search for it. Maybe it has the power to destroy both universes if misused.