A possible scenario for a Buffy Dark Angel crossover fanfiction is that there is a portal that opens up between Sunnydale and the post - apocalyptic world of Dark Angel. Buffy and her friends are pulled through the portal and find themselves in a strange new world. They quickly realize that they need to work with the locals, especially Max and her group of transgenics. The vampires start to see the transgenics as a new source of food or a threat to their dominance. Buffy has to use her slayer skills and work with Max's combat abilities to close the portal and stop the chaos that is ensuing.
One idea could be that the vampires in Buffy's world somehow cross over to the Dark Angel universe and the transgenic characters have to deal with them. They might initially mistake vampires for some new kind of transgenic experiment gone wrong. Another idea is that Buffy herself travels to the Dark Angel world and teams up with Max to fight against a common enemy that is threatening both of their worlds, like a powerful demon that has found a way to use transgenic technology for evil purposes.
In some 'dark angel buffy crossover fanfiction', the characters' backstories are explored in relation to each other. Maybe Buffy discovers that there are some similarities between her calling as a slayer and Max's situation as a genetically enhanced being. They could bond over their sense of being different from the normal humans around them and their responsibility to protect those who can't protect themselves. This type of fanfiction often delves deep into the emotional and psychological aspects of the characters, making it a more complex and interesting read.
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 a case in NCIS leads them to discover a secret project related to the genetic experiments in Dark Angel. Maybe they find a link between a murdered Navy officer and a group trying to recreate the transgenic experiments. So, the NCIS team has to work with Max from Dark Angel, who has her own knowledge and skills in this area.
In my opinion, Faith from Buffy and Alec from Dark Angel would be great to focus on. Faith has her own troubled past and her unique slayer abilities. Alec, on the other hand, is a transgenic with a bit of a roguish charm. The two of them could have a really interesting dynamic. Maybe they start off as rivals, not trusting each other, but as they face a common enemy, they start to understand and respect each other. And of course, Buffy and Max are also must - haves. They are the main characters of their respective shows and their leadership skills and different outlooks on life would make for a great story.
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.
Fanfiction.net is also a good place to search. There are many different categories and tags, so you can specifically look for crossovers between 'Dark Angel' and 'Buffy'. Just be prepared to sift through a lot of stories to find the ones that interest you the most.
One popular one could be 'Soulmates Across Dimensions'. It beautifully weaves the two universes together, exploring how Buffy and Angel's relationship might develop further in new and unexpected ways. It has some great action scenes and emotional moments.