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.
There's also 'Slayers and Witches United'. This fanfic focuses on Willow from Buffy and Piper from Charmed forming a strong bond. They share their knowledge of magic and work together to solve a mystery that has been plaguing both Sunnydale and the Halliwell Manor. It has some really cool action scenes and great dialogue between the characters.
I really liked a fanfic called 'Power Convergence'. In this story, the Charmed Ones end up in Smallville and they have to figure out how to get back home. Along the way, they meet Clark and his friends. It's a great read because it shows how the different power dynamics from each show would play out when the two worlds collide.
A popular one is 'Crossing Paths'. This fanfiction has an interesting take on the crossover. It starts with a mystery that involves both the Supernatural and Buffy universes. The author does a great job of writing the characters true to their original personalities. Buffy's determination and the Winchesters' tough - guy attitudes are on full display, and the story builds up to a very satisfying climax.
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.
I like 'Buffy in the SGC'. In this fanfic, Buffy is somehow transported to the Stargate Command. The author does a good job of showing how Buffy's slayer skills are both useful and a bit out of place in the military - science - fiction setting of Stargate. There are some really cool fight scenes against Goa'uld with Buffy leading the charge.
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.
There's also 'Bones Meets Buffy'. In this fanfic, the two universes collide when a series of murders in Buffy's Sunnydale seem to have a connection to some archaeological finds that Bones is researching. It's a fast - paced story that keeps you on your toes. The author does a good job of maintaining the personalities of both sets of characters. For example, Bones is as logical and matter - of - fact as ever, while Buffy is her brave and sassy self.
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.
A must - read vampire Buffy fanfiction is 'Shadows of the Night'. This one shows Buffy dealing with a new breed of vampires that are much more cunning and powerful. The author does an excellent job of building the tension throughout the story.
I really liked 'When Hunters Meet Slayers'. It's a well - written fanfic that does a good job of melding the two universes. It has some great action scenes where Sam uses his research skills to help Buffy and her gang figure out a new type of demon that has emerged, which is a combination of a demon from 'Supernatural' and one from 'Buffy'.
One great Buffy and Sirius Black fanfiction is 'The Crossover of Two Worlds'. It beautifully combines the magical elements from the Harry Potter universe where Sirius Black belongs and the unique vampire - slaying world of Buffy. The story has an engaging plot that keeps you hooked from the start as it weaves their two different storylines together in a very creative way.