One idea could be that Doctor Strange accidentally opens a portal to the wizarding world of Harry Potter. He could end up at Hogwarts and be mistaken for a new kind of wizard. Maybe he tries to use his magic to teach the students some new spells or ways of magic, but it causes chaos because his magic is so different from theirs. For example, his use of the Time Stone could mess with the time - turners at Hogwarts.
One idea could be that Sherlock's deductive skills are needed to solve a mystery in the magical world of Doctor Strange. Maybe a powerful artifact has been stolen and Sherlock's unique way of seeing details helps find it. Another could be that Doctor Strange uses his magic to transport Sherlock to different dimensions to solve a case that spans across realities. For example, a serial killer who can move between dimensions and Sherlock has to use Strange's magic to track him down.
One idea could be that Harry discovers he has the power to time - travel in a very unique way, not like the Time - Turner. He can enter different eras of the wizarding world's history and has to solve mysteries that are linked to his present. Another could be that he has a strange power of telepathy but only with dark creatures, which he uses to negotiate peace treaties. Maybe he also has the power to manipulate elements in a way no other wizard has, like creating new magical plants by combining the powers of fire and water.
One idea could be that the Plague Doctor arrives in the wizarding world during a mysterious illness outbreak. He might have unique magical remedies that are different from what the wizards are used to. Maybe he teams up with Harry Potter to find the source of the disease, which turns out to be a dark magic - created pathogen.
Well, there could be a story where a powerful dark magic object from the wizarding world is brought to Sunnydale. Buffy discovers it and realizes it has the power to disrupt the balance between good and evil in her world. She then travels to Hogwarts to seek help from Harry Potter. Along the way, she meets various magical creatures and has to navigate the wizarding world's politics. Once she meets Harry, they discover that there is a common enemy behind all this, someone who wants to use the chaos in both worlds for their own gain, and they must work together to stop this mastermind.
One idea could be a time - travel plot where a character from Legacies goes back in time to the era of Harry Potter. They could interact with the young Harry and his friends, and perhaps help in the fight against Voldemort in unexpected ways. For example, they might introduce a new spell from the future that gives the Order of the Phoenix an edge.
One idea could be that the Grimm creatures start to appear in the wizarding world. Harry and his friends have to use both magic and knowledge of Grimm lore to fight them. For example, they might discover that some dark magic has opened a portal between the two universes.
One idea could be that the Order of the Phoenix teams up with the Kingsman. The wizards could use some of the Kingsman's high - tech gadgets and fighting skills, while the Kingsman could learn about magic. For example, Harry Potter could be trained by a Kingsman agent in hand - to - hand combat, which would give him an edge in battles against Death Eaters. Another plot could involve a villain who is a hybrid of a dark wizard and a criminal mastermind that the Kingsman usually deal with. The two groups would have to combine their resources to stop this new threat.
Maybe some of the Hogwarts students are sent to the Naruto universe. They would be amazed by the ninja techniques. Hermione, for instance, would be eager to study how chakra works and might even find a way to combine it with magic. Harry could learn some ninja stealth skills which would be useful in his fight against Voldemort.
One idea could be that the Hogwarts students discover Halo technology. Maybe Harry and his friends find a crashed UNSC ship and start to study its advanced weaponry and shields. They could try to adapt the technology to defend Hogwarts from Voldemort. Another plot could involve Master Chief being sent to the wizarding world by accident. He then has to navigate the strange new world of magic while also dealing with threats that his usual Halo combat skills might not be fully equipped for.
In a crossover, Merida could teach the Hogwarts students about her own form of magic, like the spells she uses for her archery and how she communicates with the animals in her world. At the same time, she could learn about the complex spells and potions of the wizarding world. This exchange of knowledge could lead to some exciting adventures, like using a combination of both types of magic to defeat a common enemy.