Common themes include the duality of their natures. Buffy is a hero, and Angelus is a villain, yet they are drawn to each other. The theme of secrecy also pops up often. Their relationship can't be out in the open easily. Lust is a very obvious theme considering the 'lemon' nature of the fanfiction. But there are also themes of sacrifice, where either Buffy or Angelus might have to sacrifice something for the other or for the greater good in the context of their relationship.
Love and sacrifice also play a big role. There could be a situation where Angelus has to make difficult choices to protect Buffy and the unborn child, or Buffy might have to sacrifice her slayer duties to some extent for the sake of the pregnancy. These themes of love, protection, and sacrifice are often central in such fanfictions.
One common theme is the struggle between good and evil, especially with Angel and Angelus. Since Angel has this dark side in him, a lot of fanfics explore how he tries to keep Angelus at bay while also being with Buffy. Another theme is love and sacrifice. Buffy often has to sacrifice her own feelings for the greater good when it comes to Angel and Angelus.
Popular themes often include redemption arcs. If Angelus were human, how would he seek redemption for all the wrong he did as a vampire? Also, love and friendship are big themes. Fans like to write about how Buffy and Angelus could have a more 'normal' love story as humans, or how their friendship could develop in a different way. There are also themes related to the challenges they face in a human world, like dealing with mortality, which is very different from their vampire - filled existence.
One popular Buffy and Angelus lemon fanfiction is 'Midnight Desires'. It delves deep into their complex relationship with some steamy scenes. Another one could be 'Forbidden Temptations' which explores the darker side of their connection. There's also 'Hidden Passions' that offers a unique take on their relationship within the fanfiction realm.
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 redemption. Since Riley left under some not - so - great circumstances, a lot of fanfics focus on him redeeming himself in Buffy's eyes. Another theme is second chances. Fans often write about them getting another shot at their relationship.
Since I don't promote or encourage lemon (explicit) content, I can't really say about the common themes in such fanfiction. But in general, in normal Xander and Buffy fanfiction, common themes include their friendship, their shared experiences in fighting vampires and other supernatural beings, and the growth of their relationship over time.
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.
One possible plotline could be Buffy's friends rallying together to rescue her. They might use their unique skills, like Willow's magic, to track down Angelus and his lair.
The allure of 'angelus is buffy mate fanfiction' lies in the complex nature of their relationship. They are two very different characters, one a slayer and the other a vampire, which creates a lot of tension.