Perhaps an Imperial Arm somehow gets transported into Hogwarts. The students, including Harry, start to notice strange things happening. Harry, being the curious and brave one, decides to investigate. He finds that the Imperial Arm has powers that can either help or harm the wizarding community, and he has to make a choice on what to do with it. As he delves deeper, he might learn about a connection between the wizarding world and the origin of the Imperial Arm.
One possible plot could be that Harry Potter discovers a magical portal that leads him to the underwater world of Aquaman. There, he has to learn the ways of the underwater society while also using his magic to help Aquaman fight against an evil force that threatens both the wizarding world and the underwater realm. Maybe a dark wizard has teamed up with a sea - based villain, and Harry's unique skills are needed to defeat them.
A plot might involve Harry Potter being sent to the underworld on a quest. Hades mistakes him for a threat at first. But as Harry shows his courage and resourcefulness, Hades starts to respect him. They could end up working together to stop a dark force that is threatening both the wizarding world and the underworld. Harry could teach Hades some magic spells, and Hades could show Harry the hidden powers of the underworld.
One possible plot could be that Alduin somehow enters the Harry Potter universe. Maybe he is summoned by a dark wizard who wants to use his power to take over Hogwarts. Harry and his friends then have to figure out how to send Alduin back to his own realm while also dealing with the chaos the dragon has brought to the school.
One possible plot could be that Boromir somehow gets transported to the wizarding world of Harry Potter. He could be mistaken for a Muggle at first, but his warrior skills and strong sense of honor quickly earn him respect. Maybe he joins the Order of the Phoenix and uses his combat experience to fight against the Death Eaters. He might also form an unlikely friendship with Harry, learning about magic and teaching Harry about his own world's combat strategies in return.
One possible plot could be that Voldemort uses the dark magic to create a plague similar to the one in The Walking Dead. The wizards and witches have to not only fight against the undead but also Voldemort's forces. Harry and his friends would need to find a way to stop the spread of the plague and defeat Voldemort once and for all, while also learning how to survive in a world filled with the walking dead.
One possible plot could be that some of the Hogwarts students accidentally end up in Hell (the setting of Hazbin Hotel). They could encounter the characters from Hazbin Hotel and have to find a way to get back. Maybe they initially think it's a part of some new magical realm or a powerful dark wizard's creation. For example, Harry Potter, Hermione, and Ron could meet Charlie and learn about her plan to rehabilitate sinners. The Hogwarts students might be skeptical at first but then start to see some similarities between the way magic is used in Hell and at Hogwarts.
One possible plot could be that Voldemort teams up with Loki. The Avengers and the Hogwarts crew would then have to work together. Harry, with his knowledge of magic, could teach the Avengers some basic spells to counter the dark magic Voldemort and Loki are using. Iron Man could use his technology to enhance some of the Hogwarts defenses. And Thor could share his knowledge of Asgardian magic with the wizards at Hogwarts.
One idea could be that the vampires in Twilight somehow find their way into the wizarding world of Harry Potter. Maybe they are initially mistaken for dark creatures and there's a big misunderstanding. Harry and his friends have to figure out that they are not really a threat like the Death Eaters. They could then team up to fight a common enemy, like a new breed of dark magic - using the powers of both wizards and vampires.
One possible plot could be that Voldemort's dark magic starts to seep into Gotham City. Batman, being the detective he is, starts to notice strange and magical happenings. Meanwhile, Harry Potter, Ron and Hermione get transported to Gotham through a magical accident. They team up with Batman to stop Voldemort's influence from spreading further. Harry uses his magic skills and Batman uses his gadgets and combat skills to take on the Death Eaters and Gotham's villains who have been corrupted by the dark magic.
One possible plot could be that a magical accident transports Sakura into the Harry Potter universe. She could use her ninja skills, like her chakra control, to help Harry and his friends in a new kind of battle. For example, in a fight against dark wizards, Sakura's medical ninjutsu could be used to heal the injured wizards quickly, which is something not common in the wizarding world. And her physical combat skills might also come in handy when they face opponents up close.