One common theme is mentorship. Arthur often takes on a mentorship role for Harry, guiding him in the wizarding world. Another is family. Since Harry lacks a proper family, Arthur can represent a father - like figure in these stories.
Well, the theme of discovery is also quite common. Harry and Arthur might discover new magical things together, or Harry might discover new aspects of the wizarding world through Arthur. Also, the theme of friendship is present. Even though there is an age gap, they form a unique friendship in many fanfictions. For example, in some stories, they bond over their shared love for Muggle - related things, which is really interesting as it shows how they can find common ground despite their different backgrounds.
Friendship and rivalry are also prominent themes. In the original stories, Harry and Draco had a complex relationship. Now, in this new setup, that relationship gets a fresh take. They may have to work together more closely like Merlin and Arthur did, but their old grudges and differences still play a part, creating an interesting blend of friendship - building and ongoing rivalry.
One common theme is the exploration of forbidden love. Since Arthur, Harry, and Merlin come from different backgrounds or even different fictional universes in some crossovers, their love is often seen as something that goes against the norm. Another theme is the power of magic in relation to their relationships. Magic can either bring them together or create obstacles. For example, Merlin's magic might be a secret that affects how he can be with Arthur or Harry in a yaoi relationship.
Family is a very common theme. Since Arthur is a family man, many fanfictions explore his relationship with his wife Molly and their children. They might show how he deals with family problems or how he supports his family during difficult times.
Family and loyalty are also common themes. Arthur's role within his family in the original series was important, and in fanfiction, it can be either a supporting factor or a point of conflict in the new relationships. For example, how his new relationship might affect his relationship with his children or his wife. Also, the theme of magic and its role in these new relationships. Magic can be a way to bring the characters together or a source of problems. It can be used to show how Arthur's character adapts to different magical situations within the context of the new relationship.
One common theme is the discovery of heritage. Harry finding out he has this connection to King Arthur and then trying to understand what it means for him.
As lemon fanfiction is not something we should promote, in normal Arthur Weasley fanfictions, a theme could be his work at the Ministry of Magic. How he tries to balance his job with his family life. Also, his relationship with his wife, Molly Weasley. How they support each other, and their joint efforts in raising their children, which are all positive and family - oriented themes.
Another theme is the exploration of their different personalities. Arthur is usually more responsible and DW can be more carefree. Fanfictions play with this by putting them in situations where their personalities clash at first but then they find a way to complement each other. For instance, in a story where they are planning a party for their parents, their different approaches initially cause problems but in the end lead to a great celebration.
One common theme could be emotional pain. Maybe Arthur has been betrayed by someone close to him, and the fanfiction explores his journey of dealing with that hurt. For example, if a friend he trusted turned out to be a traitor.
One of the themes is leadership. Since Arthur is a king, many fanfictions explore how Gwen influences his leadership style and vice versa. They are often depicted as a power couple, making decisions together for the betterment of their people. Also, the theme of sacrifice is present in some fanfictions. Either Arthur or Gwen might sacrifice something for the other's well - being or for the good of the kingdom.
One common theme is the struggle between good and evil within their relationship. Since Morgana often turns to the dark side, it's about how Arthur tries to bring her back or how their relationship is affected. Another theme is love, either a forbidden love or a love that could have been if circumstances were different.