Maybe Mordred accidentally travels to the Naruto world and gets mistaken for an enemy at first. But then she meets Naruto, who sees the good in her despite her rough exterior. Together, they set out to stop a group of rogue ninjas who are trying to use a dark jutsu that has the potential to destroy both their worlds. Mordred's skills and Naruto's determination would make for an exciting combination.
A plot could be that Mordred is summoned by a powerful ninja as a last - ditch effort to fight against an ancient evil that has emerged in the Naruto world. This evil has powers similar to some of the powerful Servants in Fate Stay Night, so Mordred's expertise is needed.
One idea could be to have Naruto characters participate in the Holy Grail War. For example, Naruto himself could be a Servant, perhaps with his chakra being a unique form of mana. His skills like the Rasengan could be reimagined as powerful Noble Phantasms. Another idea is to put the Fate Stay Night characters in the Naruto world. They might have to deal with things like ninja clans and the politics of the hidden villages while also trying to find a way back to their own world.
Well, perhaps the Pokeballs in this fanfic could be like the Command Spells in Fate Stay Night. A trainer might use a Pokeball to summon and control a powerful 'Pokemon - Servant' hybrid. And the different regions in the Pokemon world could be the battlefields for the Grail War. The trainers would travel around, catching and training their unique Pokemon - Servants while competing for the Grail.
Another idea is to create a story where the rules of the Grail War are changed. For instance, instead of seven Servants, there are only three, and they have to complete different tasks to win. This would completely change the dynamics of the story and lead to new and exciting interactions between the characters.
One interesting plot idea could be that the Servants from Fate Stay Night end up in the Naruto world. For example, Saber could team up with Naruto. Saber's noble and just nature would blend well with Naruto's determination to protect his village. They could face off against common enemies like Orochimaru, who might be interested in the power of the Servants. Another idea is that the concept of chakra in the Naruto world could interact with the magic in Fate Stay Night. Maybe a mage from the Fate universe tries to study chakra and finds a way to combine it with their own magic system, creating a whole new type of power. A third plot could involve the Holy Grail War taking place in the Hidden Leaf Village. The ninjas would be both intrigued and cautious about this strange new event, and the Servants would have to navigate the politics and unique environment of the village while fighting for the Grail.
One way could be to create a crossover universe where Mordred from Fate Stay Night somehow ends up in the Naruto world. Maybe she could be a new kind of ninja with her own unique abilities based on her Fate Stay Night powers. For example, her sword skills could be translated into a new form of kenjutsu in the Naruto world.
One idea could be exploring Archer's internal struggle more deeply. Maybe show his memories of past battles and how they affect his decisions in the Fourth War. Another could be to create a scenario where Archer has to team up with an unexpected character from the Fate universe.
One interesting idea could be exploring different routes for the characters. For example, what if Shirou had chosen a different Servant at the start? Another could be delving deeper into the backstories of the Servants, like revealing more about their lives before they became heroic spirits. Some fanfictions also play with the concept of alternate universes, where the rules of the Holy Grail War are changed completely.
Another interesting plot might be a 'what if' scenario. What if Shirou had to enter Rider's world, like the world she came from before being summoned in Fate Stay Night? He would be completely out of his element, and Rider would have to be his guide. This could lead to many comical and heart - warming moments as Shirou tries to adapt and they grow closer.
One idea could be that Saber from 'Fate Stay Night' gets transported to the world of 'Frozen'. She could be mistaken for a powerful sorceress. Elsa might initially be wary of her but then realize they both have a connection to magic that they need to control. Another plot could involve Shirou being in Arendelle and using his skills to help build things, while also getting caught up in the drama between Anna and Hans. Maybe he could use his projection magic to create something that helps save the day in Arendelle.
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.