The BTVS writers kept the cast limited, sometimes to the point of repurposing minor/guest characters. Spike is the main example, and Marsters has often said he was hired to be a disposable Big Bad, then they used him to replace Cordelia. Jonathan went from helpless to nemesis, and they did something similar with Harmony, although with intended hilarity. 

Fanfic stories are not limited by time or money and don't have to work with limited casts.  There's a long history of Orginal Characters, usually to serve the action.  I'd like to see more stories where the original characters are there to make Buffy and Spike deeper as characters, and perhaps show other dimesions from their usual roles and actions in canon.

Many writers already do this as part of their fics. Years ago, there was a fic about Buffy and Spike's relationship from the viewpoin of a woman who waited tables at a coffeeshop, I think the Espresso Pump. Initially she expressed  concern for Buffy's safety, then she saw their relationship evolve.  In The Chiaroscuro Series by Miss Murchison ,  Spike's accidental intervention with an old lady evolves into interactions with a group of other people.  the_big_bad is now 106 chapters into The Neighbor's Point of View  which is exactly as title, with an original character who move next door on Revello with her husband, who gives her perspective on everyone as she becomes increasingly involved. In  Forgiveness Doesn't Come Easy   now at 31 chapters, slaymesoftly makes great use of original characters and concepts.  Recently I read two great fics (and I'm so sorry the titles and authors escape me) where original characters gave depth and context, one where Spike worked at the library, including some terrific comments from his supervisor, and another where Spike took Buffy to his British farm and she got help from his friends. Rahira wrote what I beleive she said was intended as a riff on Mary Sues that involved an OC who travels to the past to give Joyvce a message from Dawn and Buffy, which also supports Spike.  There's also another memorable fic that looked at Buffy and Spike from the perspective of a multitude of people in their lives, and I'm sorry I don't recall that title or author, and if original characters were used as well as the main characters. There are many other stories that also use original characters to great effect. I want more!

These Original Characters could be also be friends, which the core group is sorely lacking. It's understandable, given that they were created to be social misfits, and their lives lead to secrecy.  Giles is the only one who is portrayed with a circle of friends, albeit in the past. Other than Willow and Xander's firend, the late Jesse,  the only Scooby who seems to have friends outside the group is Oz, who mentions Devon and the rest of his band. Anya had one friend, the dual purpose Halfrek/Cecily.   Xander mentions work friends, but they don't seem to socialize.  If Tara has other friends, we never see them, Willow dismisses the wicca gorup as wannabes, Riley's friends/frat mates are his Initiative colleagues, and Buffy's other prinmary college connections are disposable romantic interests and the short-lived Kathy.  Other than the two kids in the basement and the boy in her art class, Dawn's only social connnection seems to be her friend Janice, who is mentioned five times and seen only once. And it seems Spike's only friend is Clem, although he does have poker regulars.  

The challenge is to write original characters who are not enemies and feasibly would be aware of and involved with Buffy and Spike. How much interaction they have is up to you, and it's your choice as to whether or not they have meaningful insights, are oblivious, or very confused. It can be serious or funny. The point is to get a fresh perspective on Buffy and Spike, and to give Buffy and Spike the settings and opportunities to show sides of themselves and/or character development  we might not otherwise see.