Arthur is definitely one. His relationship with Merlin is a key part of the stories, and seeing his perspective on Merlin's magic can be really engaging. Another is Morgana. Her complex personality and her own magical abilities add a lot of depth to the fanfiction.
Lancelot is quite fascinating in AOC Merlin fanfiction. His chivalry and his relationship with both Merlin and the other knights make for great storylines. There are also some minor characters that can become really interesting, like the various magical creatures that Merlin might encounter. They can add an element of mystery and wonder to the stories.
One interesting original character is often a powerful mage who comes to Camelot to either help or challenge Merlin. This character might have a different style of magic that Merlin has never seen before. Another could be a mysterious traveler who has knowledge about Merlin's true destiny but is reluctant to share it all at once. There are also original characters who are commoners with hidden magical abilities that Merlin discovers and decides to train.
Well, a common theme is Merlin's growth as a wizard within the AOC setting. This can involve him learning new spells and facing challenges that test his magical abilities. Also, his role in the political and social aspects of the AOC world is often explored. For example, how he might influence the court with his magic and wisdom.
There could be a new love interest for Merlin. This character might be a powerful sorceress who initially clashes with Merlin but then falls in love with him. She could have her own set of powers and a backstory that is intertwined with Merlin's new journey. Also, there might be an old character from the original series who has also been reborn in some way, like Morgana. Her new incarnation could have a different role, either as an ally or a more complex antagonist to Merlin.
Merlin himself is of course very interesting. His complex character with his magic and his loyalty. Morgana is also quite fascinating as she has her own struggles and powers.
In some cases, there might be new original characters created specifically for the fanfiction. These could be characters who have a connection to the ancient magic of Merlin and Morgana. For example, a mysterious wizard who has been guarding an artifact related to their power. They could initially be an enigma to Harry and his friends, but as the story progresses, they become an important part of the plot, either as an ally or an adversary.
Gwaine is also well - liked. He can be the fun - loving friend in modern fanfiction, always getting Arthur and Merlin into (and out of) trouble, whether it's at a party or during some kind of modern - day quest.
Merlin himself is extremely popular. After all, the fanfiction is centered around him. Arthur is also very popular as his relationship with Merlin is a major aspect of the original series and fanfictions often explore it further.
Nick Mallory is a very popular character to focus on. His journey and growth throughout the original story make him an interesting subject for fanfiction. You can explore his further adventures or what he might have been like before the events of the main story.
Obviously Merlin is a very popular choice. His magic and his role as the hidden protector of Arthur make him a central and interesting character. Writers can explore his inner struggles with his magic and his loyalty to Camelot.
Merlin himself is really interesting. His dual nature of being a normal servant and a powerful magician gives a lot to write about. Then there's Arthur. His journey from a young, somewhat arrogant prince to a great king is full of potential for stories.