The theme of loyalty could also be prominent. Arthur's loyalty to his kingdom and his knights might be tested against his loyalty to his love. He has to decide whether he will stand by his love in the face of opposition from his court or if he will side with his traditional allies and duties.
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.
Well, it could start with a chance encounter. Maybe you're in a magical forest and Arthur Pendragon rides by on his horse. Your eyes meet and there's an instant spark. It's like something out of a fairy tale.
One challenge could be the difference in your status. Arthur is a king or a great leader, and you might be an ordinary person. Society's expectations could put pressure on your relationship.
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 fanfiction that stands out is 'Arthur and Merlin: A Different Path'. This one takes a different turn from the original storyline. It imagines a world where Arthur discovers Merlin's magic much earlier. This changes the whole dynamic between them, and they face different challenges together. There is also 'The Adventures of Merlin and Arthur in the Enchanted Forest'. It's full of exciting quests and the development of their friendship.
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.
One great fanfiction is 'The Once and Future King's Heart'. It beautifully delves into the deep bond between Merlin and Arthur. It shows how their relationship evolves over time, with Merlin's magic playing an important role in protecting Arthur while also creating some interesting conflicts.
The theme of redemption also pops up. If Arthur is placed in a darker or more morally ambiguous world, his quest for justice and honor can be seen as a form of redemption for the new world he's in, or it can be a test of his own values as he faces new challenges.