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.
Faith and Phoebe would be a fascinating pair. Faith has her dark side and her complex relationship with being a slayer, while Phoebe is the more free - spirited Charmed One. They could either clash horribly or end up learning a great deal from each other, especially when it comes to matters of redemption and self - discovery.
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 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.
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.
Definitely the interactions between the two versions of Leonardo. The 2012 Leo is more light - hearted in some ways compared to the very serious 2003 Leo. They could have some great moments of leadership clashes and then learning from each other. For example, the 2003 Leo could teach the 2012 Leo some of his more stoic leadership techniques.
I think the interaction between Joker (from Persona 5) and Yu (from Persona 4) would be really interesting. They are both leaders of their respective groups, so they could exchange leadership strategies and learn from each other. Also, Morgana and Teddie would be hilarious together. Morgana's serious yet sometimes clueless nature and Teddie's over - the - top and comical personality would create some great comedic moments.
Definitely Joker and Makoto Yuki. Joker is the leader of the Phantom Thieves, very confident and strategic, while Makoto Yuki is a strong and silent type. Their leadership styles could clash or complement each other.
Merlin and Piper could be an interesting pairing. Piper has her power of freezing time and Merlin has his powerful magic. They could work together to stop an enemy. For example, Merlin could use his magic to distract the enemy while Piper freezes time to get in a decisive strike.
Definitely Piper and Dean. Piper's power to freeze time could be really useful when Dean is in a tight spot during a hunt. And Dean's no - nonsense attitude could balance out Piper's more emotional side.
Castiel and Paige Matthews could also make for a great pairing. Castiel's otherworldly, angelic nature and Paige's half - witch, half - white - lighter abilities would lead to some unique interactions. They might struggle to understand each other at first, but as they learn more about their respective powers and purposes, they could form a strong alliance to combat a powerful enemy that neither of their worlds could handle alone.
Edward and Phoebe would be a fascinating pair. Edward's mind - reading ability and Phoebe's premonition power could lead to some intense and interesting interactions. They could also use their powers together to predict and prevent disasters in the crossover world.