Love or at least a strong connection can be a theme. Some fanfictions explore a romantic connection between them, while others focus on a deep friendship or a mutual respect that develops between Merlin and Cenred, which is different from their relationship in the show.
Betrayal is another theme. Given their different allegiances in the original series, fanfictions often explore the idea of one betraying the other or the possibility of trust in spite of their differences. For example, in some stories, Cenred may seem to betray Merlin but there's a deeper reason behind it.
Some Cenred Merlin fanfictions focus on a redemption arc for Cenred. He might start as a villain but through his interactions with Merlin, he gradually changes and becomes a more positive character, perhaps even an ally to Arthur and Merlin in the end.
Fanfiction.net is also a great place to search. There are many different categories and tags that can help you narrow down your search to find the Cenred Merlin fanfiction you're interested in. However, the site's interface might be a bit more dated compared to AO3.
Sure. 'The Path of Enemies' is a great one. It explores the complex relationship between Merlin and Cenred in an alternate universe setting. The story has a lot of action and political intrigue.
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.
The theme of sacrifice is also quite prevalent. In many fanfictions, Merlin's father has made sacrifices for Merlin's future, which Merlin discovers as the story progresses. It gives a deeper understanding of their relationship.
In Merlin vampire fanfiction, the theme of acceptance is quite common. Whether it's Merlin accepting the existence of vampires or the vampires accepting Merlin's power. Another is the theme of transformation. Merlin might transform in some way due to his encounters with vampires, or the vampires themselves may transform, either physically or in their attitudes. And the theme of forbidden knowledge often pops up, as the vampires may possess knowledge that is both dangerous and alluring to Merlin.
Another theme is friendship. The neckerchief can be a focal point for the relationships Merlin has with Arthur, Gwen, and the other characters. For example, it might be a gift from a friend that holds special meaning.
Another theme is self - discovery. When Merlin crossdresses, he often discovers new aspects of himself or how others perceive him differently. For instance, he might learn that he can be more confident in a different guise. Also, it can be about breaking gender norms. In these fanfictions, Merlin challenges the traditional gender roles of the Camelot society, which adds an interesting layer to the story.
One common theme is the shock and disbelief among the characters. Given Merlin is male in the original series, his pregnancy is quite unexpected, so characters often struggle to come to terms with it.