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 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.
Maybe Arthur Pendragon gets transported to the Fate world and his presence disrupts the normal balance of power. The mages in the Fate universe might see him as a threat or an opportunity. He could end up being a wild card in the Grail War, not bound by the normal rules of Servant summoning. His noble phantasm, Excalibur, could be a game - changer in battles against other powerful Servants.
The Duke Pendragon in the novel is often portrayed as a strong and fearless leader. He is usually skilled in combat, which helps him defend his land and people.
It's about a series of adventures. It usually involves a hero's journey through different worlds, facing various challenges and foes. It's full of action and mystery.
Pendragon Visual Novel is often about the Arthurian legends. It might include the adventures of King Arthur, his knights, and the magical elements associated with that era. It can have various storylines that explore themes like chivalry, love, and power struggles.
As far as I know, there are no widely known 'pendragon muscle woman stories' in the mainstream. It could be a very specific or local concept that hasn't gained broader popularity.
Another aspect to consider in this fanfiction is the cultural clash. The feudal - like society in the Naruto world and Arturia's own Arthurian era background. Arturia might find the concept of ninja villages and the way they are structured quite strange. At the same time, Naruto and his friends could be fascinated by Arturia's tales of Camelot and the Round Table.