One interesting Buffy Xena crossover fanfiction could be where they team up to fight a common supernatural foe that combines elements from both their universes, like a vampire - centaur hybrid. Another might involve Xena teaching Buffy some of her combat skills, which Buffy then uses in a big battle against a powerful demon in Sunnydale.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created stories, and you can use the search function to specifically look for Buffy Xena crossovers. Another option is FanFiction.net, which also has a wide range of fanfiction, though the interface might be a bit different.
One interesting Buffy Angel crossover fanfiction could be a story where Buffy and Angel have to team up to fight a new, more powerful vampire threat that is immune to their usual methods. This forces them to rely on each other in new ways and re - explore their relationship.
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.
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.
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.
One interesting concept could be Buffy's slayer powers combined with Batman's detective skills. For example, they could team up to solve a mystery that involves both the supernatural elements Buffy is used to and the criminal underworld that Batman patrols. Maybe a powerful vampire is working with a Gotham crime syndicate, and their combined expertise is needed to take them down.
Well, 'Buffy and the Arrowverse' is a popular Buffy crossover fanfiction. In it, Buffy finds herself in Star City and gets involved with the Arrow and his team. She helps them take on a new type of vampire - like meta - human that is terrorizing the city. Another excellent one is 'Buffy and the Charmed Ones'. The three Halliwell sisters meet Buffy, and together they have to battle a powerful warlock who is trying to open a portal to a dimension full of demons. And 'Buffy in the Star Trek Universe' is quite interesting. Buffy is on a starship, and she has to deal with alien threats while also trying to understand the Federation's way of life, all while still being a slayer.
There are often stories where Xena has different powers or abilities in alternative fanfiction. For example, she might be able to control the elements, which leads to all sorts of new adventures and challenges for her and Gabrielle to face. It adds a new layer of excitement to the familiar characters.
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.