Loyalty also plays a role. Their possessiveness can be seen as an extreme form of loyalty. They are so loyal to Merlin that they don't want to share him with any potential threats or even other people who might not understand him. This loyalty is often tested in these fanfictions, which makes for an interesting read.
Love and protection are common themes. Arthur's possessiveness often comes from his love for Merlin and his desire to protect him from harm. For example, in many fanfics, Arthur will not allow others to harm or even insult Merlin.
One common theme is loyalty. The knights' loyalty to Merlin and vice versa is often explored. It shows how they stand by each other through thick and thin.
One common theme is the exploration of forbidden love. Since Merlin is a sorcerer and the knights are bound by the laws of Camelot, their relationships often have to be kept secret. Another theme is the power dynamics, as Merlin has magic which gives him a certain kind of power, and the knights have their own physical and social power. This can create interesting tensions in the relationships.
Love, in different forms, is also a theme. It could be platonic love with a possessive edge, or it could be budding romantic love. The fanfiction may explore how this love affects their actions and decisions. For example, Arthur's possessive actions might be his way of expressing love without fully realizing it. And Merlin's reaction to it can be a way to show the growth of their relationship.
One popular one is 'The Knight's Claim'. In this fanfic, the knights show their possessiveness in a protective way. They are always by Merlin's side, not allowing any harm to come to him. It's a great read for those who like this kind of dynamic.
One common theme is protection. Edward's possessiveness often stems from his desire to protect the object of his affection. He sees himself as the one who can keep them safe from all harm.
Love and jealousy is another theme. His possessiveness can be seen as an extreme form of love, and jealousy often plays a part when he feels someone else is getting too close to his loved one. For example, if another character shows an interest in the person Edward loves, his possessive side flares up.
One common theme is his unrequited love. Snape's possessive nature often stems from his deep, unspoken love for someone, usually the protagonist. Another theme is his need for control. His possessiveness can be seen as a way to control the situation and the person he is interested in.
One common theme is the clash of different magical systems. Merlin has his druidic magic and Loki has his Asgardian magic, so a lot of fanfics explore how these two types of magic interact. For example, in some stories, they might try to combine their powers to defeat a common enemy.
In sub Merlin fanfiction, a major theme is secrecy. Given Merlin's sub position, he must keep his magic a secret, and this secrecy drives a lot of the plot. Another theme is growth. He grows as a person and as a magic - user while being in a sub role. It's about how he overcomes the limitations of his position. Additionally, the theme of sacrifice is common. Merlin is often in situations where, because of his sub role, he has to sacrifice something for the greater good, whether it's his own safety or his relationships.