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.
It could be the result of a curse or a dark magic experiment gone wrong. There might be a group of dark mages in the Fate/stay night setting who were trying to create a super - powerful servant. Their experiment involved mixing different types of magic and somehow it connected to the Naruto universe and Naruto was summoned. He would then be a very unexpected and perhaps uncontrollable factor in the world of 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.
I'm not entirely sure as it could cover a wide range of things. It might be a fan - made story that combines elements from Naruto and Fate Stay Night in a rather strange or comical ('troll') way. Maybe it has Naruto characters in the Fate Stay Night world dealing with magic and the Holy Grail War in unexpected ways.
Well, one possibility is that Naruto's unique skills and chakra would completely baffle Jeanne and her comrades. His ability to create multiple clones might be mistaken for some sort of dark magic at first. Maybe Naruto would use his Rasengan in battle against Jeanne's enemies, which would be a completely new and unheard - of form of attack in her world.
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.
If Naruto were in Fate Stay Night as a godlike being, he would likely change the power dynamics significantly. His vast chakra reserves and powerful jutsu could overpower many of the Servants. For example, his Rasengan might be able to break through even the strongest magical defenses. He could also use his shadow clone jutsu to outnumber his opponents in battle, which is a strategy not typically seen in the Fate Stay Night universe.
One common theme is the power struggle. Both Naruto and Fate Stay Night have characters with different levels of power, and in fanfiction, often there are stories about characters trying to gain more power or deal with the power they have. For example, Naruto characters might have to adapt to the magic - based power system in Fate Stay Night.
The plot might involve Naruto being summoned to the Fate Stay Night world by accident. There, he catches the attention of Fem Gilgamesh. She could be intrigued by his unique chakra and his ability to keep getting back up no matter how many times he is knocked down. They might then embark on a journey to find a way to send Naruto back home. Along the way, they face various challenges and enemies. Naruto could teach Fem Gilgamesh about the value of friendship and hard work, which are concepts not as emphasized in her world, while she could introduce Naruto to the complex magic and power hierarchies of the Fate world.
If Harry Potter were to watch Fate Stay Night fanfiction, he might start thinking about how different the rules of magic are. In his world, magic is more about spells and potions, while in Fate Stay Night there's a whole other level of magic with summoning Servants and the power struggles for the Grail. He could also get some new ideas about heroism and sacrifice from the characters in the fanfiction.
Well, it could be really interesting. He might be confused at first, seeing the different world of Fate Stay Night with its own set of rules about magic, heroes and the Holy Grail War. Maybe he'd compare the magic in his world to what he reads in the fanfiction. He could also get some new ideas about friendship and loyalty from the characters in the fanfiction.
The concept of Naruto being a Master in the Fate/stay night world is also very engaging. He could summon Servants and form unique partnerships. His understanding of loyalty and friendship from the Naruto world would influence how he interacts with his Servant. It often leads to heartwarming and action - packed storylines.