Maybe a situation where Harry Potter himself gets transported to the Charmed universe. He could be a bit lost at first, but then start to adapt and use his knowledge of magic from Hogwarts to help the Charmed ones fight their enemies. He might introduce some new magical creatures from his world to theirs.
One idea could be that the Halliwell sisters from Charmed visit Hogwarts. They could help the students learn some new types of magic that are more in line with their own brand of magic. For example, teaching them how to create protection spells that are more intuitive rather than formula - based like some Hogwarts spells.
Perhaps there could be a plot where Harry teams up with the Charmed Ones to fight a common dark force that threatens both the wizarding world and the world of the Charmed Ones. This would involve a lot of strategizing and combining their different magical abilities.
Perhaps a spell gone wrong in the Charmed universe causes a portal to open to the Twilight world. The Charmed Ones end up in Forks and meet the Cullens. They initially suspect the Cullens of being dark magic users but then realize they are vampires. Together they face a common enemy that is drawn to the combined magic and vampire allure in the area.
One idea could be that the Charmed Ones and Buffy team up to fight a powerful demon that has the ability to move between their two worlds. They have to combine their unique magical powers and slaying skills to defeat it.
One idea could be that the Charmed Ones have to team up with the Winchesters to stop a powerful demon that is causing chaos in both their worlds. Maybe the demon has a spell that can break the magical protections of the Halliwell Manor and is also targeting hunters. Another could be a crossover where Phoebe's premonition power combines with Sam's psychic abilities to foresee a major apocalypse event that they must prevent together.
One idea could be that the Halliwell sisters somehow end up in Camelot. They could use their magic to help Merlin deal with a powerful dark sorcerer that even his magic can't handle alone. Maybe they teach him some new spells or ways to control his magic better.
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.