If such a fanfic exists, possible themes could include the struggle between good and evil, as Xander is likely a 'good' character and the demon is 'evil'.
It's possible that in some more niche or edgy corners of fanfic writing, there could be something like this. However, it's not a common or widely promoted type of story. Fanfic writers usually focus on more positive relationships or at least ones that are more in the spirit of the original work.
One common theme is the internal struggle of the character having the crush. They might be dealing with feelings of insecurity or fear of rejection in relation to Xander.
One common theme is friendship. Their long - standing friendship is often explored more deeply, like how they support each other through all the chaos in Sunnydale. Another theme is unrequited love. Sometimes Xander has feelings for Buffy that she may or may not return in the fanfic. And of course, the theme of fighting together against evil. They are part of the Scooby Gang and their teamwork in battling vampires and demons is always a part of these stories.
One common theme could be friendship. Maybe the story focuses on how Buddha and Xander become friends and the adventures they have together.
One common theme is redemption. Angel, being a vampire with a soul, often has a journey of redemption, and Xander can be a part of that journey in the fanfic. For example, Xander might be the one to push Angel further towards the good side.
Survival is also a big theme. Xander has to adapt to the harsh and unforgiving world of Westeros. He faces challenges like the cold, lack of food, and enemies at every turn. Another theme is loyalty. Xander has to decide who he can trust among the many devious characters in the Game of Thrones universe, and his choices regarding loyalty greatly impact the story.
Since I don't have specific knowledge about 'boypussy xander spike' fanfic, I can only guess. It might have themes related to the relationship between the two characters. Maybe themes of loyalty, or how they face challenges together. It could also potentially have themes of growth, as the characters develop throughout the story.
Another possible theme is the struggle between good and evil within the context of their relationship. Angelus has his dark side, and Xander represents the more normal human side, so their interactions can be a study of how these two elements interact.
One common theme could be friendship. They might explore how Xander and Brian O'Connor form a strong bond, whether it's through shared interests or going through tough situations together.
One common theme is obsession. Xander's yandere nature would likely lead to an obsessive focus on Corrin. Another theme could be possessiveness, where Xander feels the need to own Corrin in a sense. Also, there might be a theme of internal conflict within Xander between his love for Corrin and his extreme actions. And perhaps the theme of Corrin's struggle for independence in the face of Xander's overbearing behavior.