Well, perhaps the Twilight vampires' unique abilities, like Edward's mind - reading, could be a threat to Buffy's team. They might need to find a way to counter it. Or, the Slayer power could be something the Cullens are intrigued by and want to study. This could lead to a story where they work together to understand Buffy's power and how it can be used against a common enemy, like a new breed of super - vampires.
One idea could be that Buffy and the Scooby Gang find themselves in Forks and have to deal with the vampire situation there. They might initially clash with the Cullens due to different ways of handling vampires. Another could be a love triangle between Buffy, Edward, and Angel. It would add a new layer of complexity to the relationships already present in both series.
Another concept could be that a powerful demon from Buffy's world is attracted to the unique vampires in Forks. This demon starts causing chaos, and Buffy and the Cullens have to team up to stop it. During this time, they learn from each other's fighting styles and supernatural knowledge. For example, the Cullens could teach Buffy more about their kind of vampire lore, while Buffy could show them some of her combat techniques against demons.
Here's an idea. Buffy and Xander end up in Lothlórien. The elves are initially suspicious but then realize they are there to help. Buffy could use her agility and strength to assist in defending the forest. In another scenario, the Nazgûl could cross over to Buffy's Sunnydale. This would be a huge threat, and Buffy would have to use her knowledge of fighting demons along with the help of her friends and the weapons of Middle - earth (if they can get them) to defeat these powerful enemies.
One idea could be that Buffy and the Highlander team up to fight a common supernatural enemy that is causing chaos in both their worlds. Maybe a powerful demon that can't be killed easily, and the Highlander's immortality and Buffy's slayer skills are needed to defeat it.
Well, for a Twilight and It crossover fanfiction, you could start with the setting. Suppose the town of Forks is somehow connected to Derry. The vampires in Twilight are often in search of new territories. What if they stumble upon Derry which has its own dark secrets in the form of Pennywise? The Losers Club could initially be terrified of the vampires, thinking they are some new form of evil. But then they realize that they might need to work together to defeat Pennywise who sees the vampires as new prey. It could be a story full of mystery, horror, and unexpected alliances.
A possible plot could involve a powerful and ancient evil that has ties to both the Buffyverse and the world of Sons of Anarchy. This would force the two groups to work together. The Sons of Anarchy members could provide muscle and their knowledge of the criminal underworld, while Buffy and her friends bring their supernatural expertise. It would be a clash of two very different worlds and cultures, but also a chance for some great character development and action.
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.
Perhaps a spell gone wrong in the Buffy universe causes some of the main characters from there to swap bodies with characters from The Vampire Diaries. This could lead to humorous and tense situations as they try to figure out how to reverse the spell while still dealing with the threats in each other's worlds.
One idea could be that Buffy and her team are transported to the Star Trek universe. Maybe they have to fight vampires that have been affected by some sort of alien technology. Another could be that a Star Trek crew member visits Sunnydale and has to adapt to the whole demon - slaying thing. They could end up helping Buffy with some high - tech knowledge from the future.
Well, perhaps the Force in the Star Wars universe is related to the magic in Buffy's world. A Jedi could sense Buffy's special power and they team up to fight a common enemy. For example, a powerful dark magic user who is causing chaos in both their universes. It could be like Buffy teaching the Jedi about the more earthly, hands - on combat she's used to, while the Jedi shows her how to use the Force in combat.
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.