Spike is often paired with Buffy in this type of fanfiction. Since they already had a complex relationship when Buffy was a slayer, it becomes even more interesting when she becomes a vampire. Angel is also a common pairing. Their past relationship and his own experience as a vampire with a soul add a lot of depth to the story.
One interesting plot twist could be that Buffy becomes a vampire but still retains some of her Slayer powers. This creates an internal struggle as she has to fight her new vampire instincts while also using her old skills. Another could be that she turns into a vampire and finds out that there's a whole new level of vampire society that she was unaware of, which changes her perspective on the whole vampire - slayer dynamic.
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.
Definitely Spike. He has his own complex relationship with Buffy. In the fanfiction, he could be a mentor of sorts to Buffy, teaching her how to be a 'better' vampire, or he could see her as a rival in the vampire world. Giles would also be around. He'd be the voice of reason, trying to figure out what caused Buffy's transformation and looking for ways to reverse it while still guiding the Scooby Gang.
One possible scenario could be that Finn, after being turned into a vampire, struggles with his new identity. He might have to deal with the urge to drink blood while still holding onto his old values. For example, he could try to find alternative ways to satisfy his thirst that don't involve harming humans, like drinking animal blood.
One idea could be that Stiles is turned into a vampire against his will. Maybe he's attacked one night while out investigating something supernatural. As a vampire, he has to struggle with his new urges, like the thirst for blood, while still trying to be the same person his friends know. He might start to see the world in a different light, both literally and metaphorically, as his vampire senses heighten.
One possible story could be that Mimiko is bitten by a mysterious vampire one night while walking alone. She then starts to experience strange cravings and enhanced senses. At first, she's terrified, but as time goes on, she begins to embrace her new vampiric nature.
Well, I really liked 'Giles: The Early Years'. It gives a great backstory to Giles, showing how he got involved with the Council and his first encounters with the supernatural. 'A Day in the Life of Giles' is a charming short fanfic that just follows Giles through an ordinary day in Sunnydale, but with his unique perspective as a Watcher. There's also 'Giles and the Ancient Artifact' which has an exciting plot involving Giles and a mysterious magical item.
One common theme is identity. Since Dawn becomes a vampire, she has to re - evaluate who she is. Another theme is family. Buffy's reaction to Dawn's new state and how they interact as sisters in this new situation is often explored.
One common theme is identity. Dawn has to figure out who she is as a vampire, different from her human self. Another theme is family. Buffy and the rest of the gang have to deal with Dawn in this new state, which tests their family bonds.