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.
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.
Well, in such a fanfiction, they could team up against a common enemy. Maybe an ancient dark force that has been awakened in both their times. Their combined knowledge of magic from different eras would be a great asset.
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.
Another possibility is a story where a vampire in The Vampire Diaries discovers an ancient relic related to Arthur Pendragon. This leads to a journey through time where they end up in Camelot. There, they face the challenges of Arthur's court, and perhaps a vampire falls in love with one of Arthur's knights or a lady in waiting. It would be a story full of adventure, romance, and the clash of two very different worlds.
One key element is the concept of courtly love. Arthur, as a king, was expected to show a certain refined and chaste form of love towards his queen, Guinevere. Another element is the conflict that love can bring. In his case, the conflict with Lancelot over Guinevere was a major part. Also, loyalty plays a role. Arthur's loyalty to his love and his kingdom were constantly being tested.
One common theme is gender role reversal. Since Arthur is typically male in the original legends, fanfictions with a female Arthur often explore how she navigates a world that has certain expectations for men. For example, she might face different challenges when it comes to leading knights or being accepted as a ruler.