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.
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.
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 great Buffy and Charmed crossover fanfiction is 'The Power of Two Worlds'. It combines the unique magic systems of both shows in an interesting way. The characters from Buffy and Charmed have to team up to fight a common enemy that threatens both their universes. The author does a good job of keeping the personalities of the original characters intact.
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 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 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.
Character development is key. When you bring together characters from two different shows, you have the opportunity to show new sides of them. For instance, how would Buffy react to the magic used by the Charmed sisters? And how would the sisters handle the presence of a slayer? The fanfiction should also have a good pacing. Not too rushed so that the interactions between the characters feel natural and not too slow that it becomes boring. A balance is important.
Piper and Xander could be really interesting. Piper with her ability to freeze and blow things up, and Xander who often provides the comic relief but also has a lot of heart. Their interactions could be a mix of Piper being exasperated by Xander's jokes but also growing to appreciate his loyalty.
One idea could be that Buffy is transported to the wizarding world. She could use her slayer skills to fight dark wizards. Maybe she teams up with Harry Potter to take on a new kind of threat that combines the powers of vampires and dark magic.