Another plot could involve Merlin traveling to the modern world where the Charmed ones live. He gets caught up in one of their battles against demons. His ancient magic and knowledge of magic from a different time could provide a unique perspective and assistance. For example, he might have a spell that can counter a demon's power in a way the Charmed ones haven't thought of.
A more complex plot could be that there is a magical artifact that has the power to destroy both Camelot and the world of the Charmed ones. Merlin and the Charmed sisters have to team up to find it before it falls into the wrong hands. Along the way, they learn about each other's magic systems and how to combine their powers effectively.
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.
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 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.
One idea could be that Merlin, with his magic from the Arthurian world, meets Eragon in Alagaesia. Maybe Merlin helps Eragon to better understand and control his own magic, as Merlin has centuries of experience. He could teach Eragon about different forms of magic and how to use it more subtly.
Perhaps a powerful enemy from Merlin's past resurfaces and is causing chaos in both his world and the world of the Originals. Merlin and the Mikaelson family have to work together to defeat this enemy. The Mikaelsons' cunning and physical strength combined with Merlin's magic could make for an epic battle. Along the way, there could be interesting character interactions, like Elijah being impressed by Merlin's wisdom and honor.
One idea could be that the Winchesters find themselves in the Charmed universe while hunting a cross - dimensional demon. They team up with the Halliwell sisters to defeat it. The contrast between the two groups' hunting styles would make for great drama.
One idea could be that Merlin uses his magic to help Harry Potter in his fight against Voldemort. Maybe Merlin's ancient magic knowledge gives Harry an edge in finding Horcruxes. Another could be that the characters from the two universes meet at Hogwarts, like Merlin being a special guest lecturer, sharing his experiences of magic in a different era.
One idea could be that Hermione, with her vast knowledge from the wizarding world of Harry Potter, travels through time or a magical portal and meets Merlin. She could help Merlin with some of his more complex magical problems using her unique spells and techniques from Hogwarts. Another plot could involve them teaming up to defeat a common enemy that threatens both their worlds, perhaps a dark magic user who has found a way to bridge the two universes.
One idea could be that Merlin uses his magic to help Sherlock solve a case that seems to have supernatural elements. For example, a series of murders where the victims all seem to have died from some unexplainable force. Merlin's ability to sense magic and his knowledge of the old ways could give Sherlock new perspectives on the case.