Some Xander adult fanfiction might also focus on emotional exploration. It could be about his inner turmoil, his fears, and how he copes with them in a more adult - oriented way, perhaps dealing with more complex moral and ethical situations.
One common theme could be character development. Writers might explore Xander's growth in different scenarios. Another could be relationships, like his friendships or romantic interests.
Another theme is self - discovery. Xander may be depicted as finding out more about his own abilities and identity. He might learn new skills or realize his importance in the group dynamic. This can be really interesting as it shows his growth as a character.
Another common theme is the clash of different mythologies. The show Xander is from might have its own set of supernatural rules and creatures, and when combined with the Supernatural world, there are a lot of interesting interactions. This can lead to stories where the characters have to figure out how to deal with new and unexpected threats that come from the combination of these two different mythologies.
Power - up is a very common theme. Xander goes from being the 'normal' one in the group to having some extraordinary powers or skills. Also, friendship is often explored. His relationships with Buffy, Willow, and the others are either strengthened or tested as he becomes a bamf. In some stories, his newfound status makes his friends see him in a whole new light, which can be both positive and negative for their friendships.
Another theme is the battle between good and evil. Angel is a vampire fighting for good, and Xander is often involved in the fight against the forces of darkness in Sunnydale. Fanfictions may show how they combine their strengths, Xander's street - smart knowledge and Angel's supernatural abilities, to combat evil.
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 frequently seen theme is the clash of different time periods' cultures. Xander has to deal with the differences between modern times and the past or future he travels to. Also, the idea of fate and free will is common. Does Xander have the power to change things, or are they already set in stone? In some fics, he struggles with this concept as he travels through time.
One common theme is the exploration of their unlikely connection. Since they are so different in the original context, fanfiction often focuses on how they bridge that gap.
In 'Game of Thrones Xander Harris fanfiction', a frequent theme is the exploration of power dynamics. Xander, being a new element in the 'Game of Thrones' world, has to navigate the complex web of power that exists among the different houses and factions. He might start as a nobody but gradually rise through the ranks, or he might be used as a pawn by those in power. Another common theme is the struggle between good and evil. Xander could be a force for good in a world full of corruption and violence, or he might be tempted by the dark side of power, much like many characters in 'Game of Thrones'. Also, the theme of identity is often explored as Xander has to figure out who he is in this new and dangerous world, while still holding on to his roots from his own world.
One common theme is the crossover aspect. Writers often explore how the two different universes (Buffyverse and Twilight - verse) collide. For example, how Xander would react to the vampire world of Twilight or how Edward would deal with the demons in Buffy's world.