One possible plotline could be Arthur trying to escape the whore house and regain his honor. He might meet some unlikely allies among the people there who help him plan his escape.
One great 'Arthur Pendragon Merlin fanfic' is 'The Once and Future King' -esque story where Merlin's magic is a key factor in Arthur uniting the kingdoms. It shows their deep bond and how Merlin uses his powers to protect Arthur in a very creative way.
A good 'Arthur Pendragon Merlin fanfic' often has a strong portrayal of their friendship. For example, it might show how they support each other through difficult times. Like when Arthur is facing a tough battle, Merlin is there with his magic to help, and vice versa.
Saber from Fate/Stay Night would be an obvious choice. Since she is also related to the Arthurian legend, there could be a lot of interesting interactions between them. They could compare their experiences as king, or even fight side by side against a common enemy.
I really like 'Avalon's Arthur in Fate Fanfic'. In this story, Arthur has a deeper connection with Avalon, his mythical resting place. It gives a fresh perspective on his powers and his relationships with other Fate characters. Then there's 'Arthur's Allies in Fate'. This fanfic focuses on Arthur forming new alliances and how those relationships shape his journey in the Fate world. 'The Pendragon Legacy in Fate' is also good. It explores how Arthur's legacy affects the present - day Fate universe and the characters within it.
One interesting Arthur Pendragon OC fanfic could be about an OC who is a powerful sorcerer from a far - off land coming to Camelot. They initially face suspicion but end up helping Arthur in a great battle, using their unique magic skills. This kind of story can add new elements to the Arthurian legend.
One popular character paired with Arthur in Fate fanfic is Saber. Since she is an incarnation of Arthur in some versions of the Fate series, they often have complex interactions. Another is Merlin. He is Arthur's mentor and advisor in the original legend, so in fanfics, they are often shown having deep conversations and Merlin guiding Arthur through different situations. Also, Artoria is commonly paired with Arthur as they are both related to the Arthurian legend and can have interesting dynamics in the Fate fanfic world.
To create a good Arthur Pendragon OC fanfic, you need to pay attention to the writing style. Use descriptive language to bring the world of Camelot to life. Describe the castle, the forests, and the people. When writing the interactions between the OC and Arthur, show rather than tell. Let the readers feel the tension, the friendship, or the conflict through their actions and dialogues. And don't forget to have a good pacing. Don't rush through important moments but also don't drag out the story unnecessarily.
One great Arthur Pendragon hurt fanfiction is 'The Wounded King' which delves into his emotional turmoil after a major battle. It shows his struggle to regain his strength both physically and mentally.