I really like 'A New Beginning'. It features an OC who is a healer. This OC becomes friends with Merlin first and through him, gets involved with Arthur. The story focuses on how the OC uses her healing skills to save Camelot during a plague. It's a really heartwarming story that shows the relationship between the OC, Merlin, and Arthur in a very positive light.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3) or FanFiction.net. Use the search filters to specifically look for Merlin fanfictions with original characters. Check the ratings and reviews of the stories as well. High - rated and well - reviewed ones are likely to be good.
Power and its consequences is another theme. The OC, if they have special powers, has to deal with how to use them. In some fanfictions, the OC might struggle with the temptation of using their power for personal gain or to change the course of Camelot's history in a way that could have unforeseen negative consequences.
One common theme is friendship. Since Gwaine and Merlin are already friends in the original series, the OC often comes in to either strengthen their friendship or create some sort of conflict within it. For example, an OC might be jealous of their bond at first.
A popular one is 'Merlin and the Kingsman Trainee'. In this fanfiction, Merlin takes on a trainee who has a lot to learn. It's interesting because it delves into Merlin's teaching style and how he imparts his wisdom and skills. There are some great character interactions and a bit of humor thrown in as well.
One great Merlin fanfiction is 'A Different Destiny'. It offers a unique take on Merlin's journey, exploring his powers in new and exciting ways. Another is 'The Once and Future King's Apprentice' which delves deep into the relationship between Merlin and Arthur in a very engaging manner. And 'Merlin's Hidden Magic' is also quite good as it presents some really creative plotlines regarding Merlin's true potential.
One great 'Merlin and Escanor' fanfiction is 'The Bond Beyond Kingdoms'. It beautifully explores their relationship in a world where they have to team up against a common magical threat. The story delves deep into their individual powers and how they complement each other.
One great Nimueh and Merlin fanfiction is 'The Enchanted Bond'. It weaves a complex story of how Nimueh and Merlin's fates are intertwined in a way that was not fully explored in the show. The author does a wonderful job of creating a believable backstory for their interactions.
One great 'Hermione and Merlin Fanfiction' is 'Time Warp of Magic' which has an interesting plot where Hermione is sent back in time and meets Merlin. The way their magic interacts is really fascinating.
One great Merlin and Gwen fanfiction is 'A Royal Bond'. It beautifully explores their relationship in a world filled with magic and political intrigue. The author does a wonderful job of portraying Gwen's strength and Merlin's loyalty.