One common theme is the struggle between their dark and light natures. Buffy being the slayer and Spike being a vampire, their relationship is filled with this contrast. Another theme could be redemption, as Spike might be seen as trying to be a better 'person' for Buffy. And of course, there's the theme of forbidden love since they are from two different worlds.
Well, the theme of forbidden love is quite common. Given their different natures (Buffy being a slayer and Spike being a vampire), their relationship is often seen as something that shouldn't be. Another theme is the power dynamic. Buffy is strong and powerful as a slayer, and Spike has his own vampiric powers. How these powers interact and how they balance their relationship around it is a recurring theme. And, of course, there's the theme of sacrifice. They are both characters who are willing to sacrifice for the greater good or for each other in many fanfictions.
There are quite a few Buffy and Spike fanfictions out there. One popular one might be 'Some Title' (if it exists). You can often find them on fanfiction websites. Just search for Buffy and Spike fanfiction and then look through the results to find ones that focus on their relationship in a romantic or loving way.
One common theme is the love triangle. The complex relationships between Buffy, Spike, and Angel are often explored, with the characters torn between different kinds of love. For example, Buffy's love for both Spike's wildness and Angel's brooding nature.
One common theme is the exploration of their forbidden love. Buffy and Spike's relationship was always a bit taboo in the original series, and in fanfiction, this is often magnified. Another theme is the power struggle between them, which adds an extra layer of tension to their passionate encounters.
One common theme is the struggle with their identities. Buffy is a slayer and Spike is a vampire, so having a baby challenges their usual roles. Another theme could be the reactions of their friends and enemies. Their friends might be shocked or supportive, while their enemies could see it as a weakness to exploit.
Love and family are common themes. The stories often revolve around how Spike and Buffy's love for each other is strengthened by their love for their child. It's about building a family in the midst of their dangerous and chaotic world.
Another theme is the battle between good and evil within Spike. Given that he's a vampire, there's always that struggle, and Buffy's influence on his choices is a big part of many fanfics. For example, some fanfics might show how Spike's love for Buffy makes him question his vampiric nature and strive to be more 'human' in his actions.
One common theme is redemption. Spike's journey towards redemption and how Buffy fits into that is often explored. Another is the idea of forbidden love, as their relationship has always had elements of being something not quite accepted. Also, the theme of sacrifice comes up a lot, like one of them sacrificing for the other's well - being or for the greater good.
A common theme is the exploration of their forbidden love. Since Spike is a vampire and Buffy is a slayer, their relationship is taboo in the Buffyverse, and fanfictions often play on this aspect. Another theme is passion and desire, which is where the 'lemon' part comes in.
A recurring theme is redemption. Spike, in particular, may use time travel as a chance to redeem himself in Buffy's eyes or for his past actions. Also, the theme of fate versus free will. Do they have the power to change their destinies through time travel, or are they bound by fate? This creates a lot of interesting conflict and drama in the fanfiction.