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.
He might become more cynical at first. Being in such a degrading situation could make him lose faith in the goodness of people.
In some fanfictions, the heir of Arthur Pendragon could have a special connection to the magical creatures in the Harry Potter universe. Since Arthur was a great king, the heir might inherit his ability to communicate or form bonds with these creatures. This connection could be crucial in solving various mysteries in the wizarding world, like finding a hidden place where a powerful dark magic object is hidden. They could use the help of these creatures to outwit the villains who are also after the same object.
One possible reason could be a misunderstanding. Maybe Merlin did something that Arthur misinterpreted as betrayal or dishonesty. For example, if Merlin was secretly helping an enemy for a greater good that Arthur wasn't aware of at the time.
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 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.
One possible plotline could be that Harry discovers he has a secret heritage as Lord Pendragon. This leads to him having new powers and responsibilities. He might have to face off against enemies from both the wizarding world and the Pendragon - related threats.
One possible plotline could be a Naruto - like character being born into House Baratheon. He might have to deal with the political intrigue and power struggles within the house, using his Naruto - inspired skills. For example, his ninja skills could help him in battles for the Iron Throne or in protecting his family members within the house.
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.