One common theme is power. Since Cornelius Sigan was a powerful sorcerer, many fanfictions explore how his power influenced Merlin or how Merlin deals with that kind of power. For example, some stories might have Merlin learning from Sigan's old spells.
You can try fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfictions on various topics, including those related to Merlin, Cornelius Sigan. Just use the search function and type in the relevant names to find some interesting reads.
One common theme is the exploration of Merlin's hidden powers. Writers often create scenarios where Merlin's good looks are combined with his extraordinary magical abilities in unexpected ways.
One common theme is the exploration of Merlin's strength in the face of torture. Writers often show how his magic and inner willpower help him endure. Another theme could be the reaction of his friends, like Arthur, when they discover what has happened to him.
One of the themes is isolation. Merlin might get hurt and then be isolated either because he doesn't want others to know about his magic or because he fears being a burden. Also, self - sacrifice is a big theme. Merlin is often written as sacrificing himself for the greater good of Camelot, which leads to him getting hurt. This can range from taking a blow meant for someone else to using his magic in a dangerous way.
One common theme is the manifestation of Merlin's hidden fears. Since he has so many secrets and a great burden, his nightmares often reflect his anxiety about being discovered or his power getting out of control.
Another theme is self - discovery. In these fanfictions, Edwin often embarks on a journey where he learns more about himself, his powers, and his place in the world. For example, in some stories, he discovers new magical abilities he didn't know he had before, which leads to his growth as a character. Also, friendship is a prevalent theme. He forms bonds with other characters, and these relationships help him in his adventures and in facing difficult situations.
A sense of powerlessness is also seen. Despite having great magic, Merlin can't always use it to change things the way he wants. This inability to fully control his situation and protect those he cares about can contribute to his depression in fanfiction.
One common theme is Merlin's growth and development of his magic powers. Since he is young in these fanfictions, a lot of stories focus on how he learns to control and use his magic better.
One common theme is the self - insert character changing the course of events in Camelot. For example, they might prevent certain tragedies from happening.
The theme of secrecy also pops up a lot. Arrow has his own secrets to keep in his fight against crime, and Merlin has to keep his magic a secret in a world that might not accept it. Fanfictions play around with how they both handle their secrets, and how these secrets can either bring them closer together or cause problems in their relationship.