One reason could be the lack of promotion. Many fanfiction writers don't have the means or knowledge to effectively promote their work. So, even if they create an interesting 'Naruto x Fate Stay Night' crossover, it may remain unnoticed.
One possible neglected fanfiction could be 'Naruto in the Fate World' which might not have gotten as much attention but has an interesting premise of Naruto being transported to the Fate Stay Night universe and having to navigate its complex magic system and the Holy Grail War. It could have some unique character interactions and battles that blend the elements of both series.
Love and friendship are also common themes. The OC might form deep bonds with the existing characters from 'Fate Stay Night'. They could fall in love with a Servant or develop a strong friendship with a fellow master. In addition, the theme of destiny is often explored. The OC's presence might change the pre - ordained destiny of the 'Fate Stay Night' world, and they have to come to terms with the consequences of their actions on the overall fate of the story.
One possible storyline could be that the dovahkiin is summoned as a Servant in the Holy Grail War. Their skills and powers from Skyrim would make them a very unique and powerful Servant. They might have to face off against other traditional Servants while also trying to figure out the rules of this new world they've been thrust into.
Yes, there are quite a few. You can find them on various fanfiction websites. Some might focus on crossovers of the characters, like how the Servants from 'Fate Stay Night' would interact with the vampires in 'Hellsing'.
A frequent theme in these fanfictions is the exploration of new identities. Kirito, being in a different world, has to re - define himself, and the Fate characters also see him in a new light. Teamwork is another big theme. They have to work together to face the challenges of the Grail War or other threats in the Fate world. And of course, the theme of heroism. Kirito's heroic nature from Sword Art Online and the heroic ideals in Fate/stay night are often compared and combined in these stories. For example, in some fanfictions, Kirito takes on the role of a hero in the Grail War, much like the Servants in Fate/stay night.
A fascinating plot aspect in such fanfiction is the idea of the Holy Grail War in the Naruto universe. Picture the ninja villages competing for the Grail. The characters from Naruto would have to form alliances and strategies similar to what happens in Fate Stay Night. Also, the concept of summoning Servants in the Naruto world. What if Naruto or Sasuke could summon a powerful Servant? How would they interact? And how would the Servant adapt to the ninja world? These are all really interesting plot elements.
There's 'Fate's Ninja'. It focuses on how the ninja skills from Naruto's world can be combined with the magic and concepts in Fate Stay Night. The story has a lot of character development for both Naruto characters in the Fate universe and vice versa, showing how they learn from each other's worlds.
Power and its limits. When Naruto characters enter the Fate universe or vice versa, they have to figure out how their powers stack up. For instance, Naruto's huge chakra reserves compared to the magic in Fate Stay Night.