One common theme is protection. Spike often ends up protecting Dawn from various threats, whether it be vampires or other supernatural beings. It shows his growth from a more self - centered character to someone who cares about Dawn's well - being.
I'm not sure about the absolute best, but some popular ones might be those who have a deep understanding of the Buffy universe. They know all the characters well, especially Dawn, and can write her in - character. For example, an author who has been following the show since it started and has a passion for Dawn's character arc.
There might be a fanfiction where Dawn travels back in time and meets a younger version of Buffy. This could lead to some really interesting interactions and changes in the timeline.
One common theme is protection. Since Spike is a powerful vampire, many fanfics have him protecting Dawn from various threats in Sunnydale. Another theme is friendship. They often form a unique bond that is different from their relationships with other characters in the show.
One great fanfiction could be 'Dawn's Discovery'. In it, Dawn finds herself in a dangerous situation and Spike is the only one who can protect her. It delves into their unique relationship, with Dawn seeing Spike in a new light and Spike realizing his role as her protector.
One great place to start looking for Buffy the Vampire Slayer Spike and Dawn fanfiction is Archive of Our Own (AO3). There are numerous stories there. For example, 'The Unlikely Bond' is a fanfic that delves into how Spike and Dawn form an unexpected friendship after a series of supernatural events in Sunnydale. It's really interesting as it shows their characters evolving in new ways outside of the main show plot.
In this type of fanfiction, a frequent theme is the re - evaluation of the vampire slayer mythology. Maybe the slayer line is seen as something that is more of a curse than a gift. Also, relationships might be re - imagined. For instance, Buffy's relationships with her friends or lovers could be presented in a much darker or more complex way than in the original series. It's all about taking the established Buffy world and turning it on its head to explore different ideas.
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 great Buffy fanfiction is 'The Initiative Redux'. It takes the concept of the Initiative from the show and expands on it in a really interesting way. It delves deeper into the experiments and the characters involved in that aspect of the Buffyverse. Another one could be 'Slayer's Bond' which focuses on the relationships between the slayers and how they support each other beyond just fighting vampires and demons. And 'A Different Sunnydale' which reimagines the town and the events in a unique and engaging way.
Characteristics could include exploring the more intimate side of the relationships that were only hinted at in the show. For example, if there was a budding romance between two characters in the show, the lemon fanfiction might take it to a more sexual level. However, this can be a controversial area as it may change the tone of the original series. Also, it might focus on the physical attraction between characters like Buffy and Spike or Buffy and Angel, but it's important to note that this is not the main focus of the official Buffy the Vampire Slayer content.