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.
A really cool plot could be that there is an ancient prophecy that involves both the Charmed line and the Slayer line. The Charmed Ones and Buffy must figure out what the prophecy means and how to fulfill it before an evil force does. They might discover that their ancestries are more intertwined than they thought, and along the way, they could form new and strong friendships.
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 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.
One idea could be that Buffy and her friends are called to New Orleans because of some strange vampire activity that seems linked to the Originals. Maybe the First Evil from Buffy's world has some connection to an ancient threat in the Originals' world, and they have to team up to stop it.
One idea could be that Buffy and the Bones team have to work together to solve a mystery involving a supernatural entity that leaves behind skeletal remains with strange markings. The Bones team's scientific expertise and Buffy's knowledge of the supernatural would create an interesting dynamic.
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 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.
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.
Well, perhaps there's a spell in the wizarding world that accidentally opens a portal to Sunnydale. Buffy stumbles through and finds herself in Hogwarts. The Hogwarts students are initially confused by her, but when they see her fighting skills against the dark arts, they start to work together. Harry could learn a lot from Buffy about facing the unknown fearlessly, and Buffy could gain knowledge about magic from the wizards.
One idea could be that Buffy and her friends are transported to the wizarding world. Buffy could use her slaying skills to fight dark creatures in Hogwarts like Dementors or werewolves. Maybe she teams up with Harry and they teach each other their unique fighting styles.