Percy's combat skills and his ability to think on his feet would make him a formidable servant. He might be intrigued by Medusa, especially if he learned about her past. He could see similarities between her situation and some of the beings he's encountered in his own world. He might try to free her from the cycle of being a servant in the Grail War, which would surely stir up a lot of drama and conflict with the other parties involved in the war.
Well, Percy might be initially shocked by Medusa's appearance. But given his heroic nature, he could try to understand her situation. Maybe he'd see her as a victim of the cruel fate in the Fate universe, rather than just a monster.
One common theme is the clash of different power systems. Percy has his own set of powers and abilities, and when he enters the Fate Stay Night world, there's a contrast and sometimes a combination with the magic and Servant powers there. Another theme is adaptation. How Percy adapts to the new rules, the new culture, and the new challenges of the Fate Stay Night universe.
One possibility could be some sort of magical interference. Maybe a mage in the Fate/stay night universe was trying a new and very risky summoning spell. They aimed for a heroic spirit but due to a miscalculation in the complex magic system, they accidentally opened a rift to the Naruto universe and Naruto got summoned instead of the intended spirit. This would create a really chaotic situation as Naruto's powers and his entire way of being are so different from the normal summoned entities in Fate/stay night.
The masters would probably be very confused at first. They are used to summoning heroic spirits with a certain set of characteristics and Naruto's appearance and abilities would be completely foreign to them. For example, Shirou might be shocked and immediately on guard, trying to figure out if Naruto is a threat or an ally.
The theme of power also emerges frequently. The different power levels and types in both universes are explored. Percy's control over water and his general demigod strength are compared and contrasted with the magical powers and combat abilities of the Fate/stay night characters. Fanfics may explore how these powers interact, whether they complement each other or create conflict.
One idea could be a crossover where the characters from Fate Stay Night end up in the world of Percy Jackson. For example, Saber could be mistaken for a demigod due to her extraordinary abilities. Another could be that the Holy Grail has some connection to the Greek gods in the Percy Jackson universe, leading to a joint quest between the two groups of characters.
A great crossover aspect in fanfiction could be the exploration of the different mythologies. Percy Jackson is based on Greek mythology mainly, while Fate Stay Night draws from various mythologies for its Servants. Fanfictions could bring together the Greek gods from Percy Jackson and the heroes from different mythologies in Fate Stay Night, creating a sort of 'mythology mash - up' where the characters have to interact and deal with the differences in their mythological backgrounds.
Sure. One great Percy Fate Stay Night fanfiction is 'Percy in the Grail War'. It has an interesting plot where Percy is thrown into the Fate Stay Night universe and has to navigate the complex rules of the Grail War. Another one is 'Percy and the Servants'. It focuses on Percy's interactions with the various Servants in a really engaging way.
There's also 'A Demigod's Fate' which combines the two worlds really well. In this fanfic, the characters from both universes interact in a way that stays true to their original personalities. For example, Percy's loyalty and his quick - thinking in battle blend well with the complex magic system of Fate/stay night.