It's really hard to say who would be the most powerful. On one hand, in Fate Stay Night, Servants like Gilgamesh have incredible power. His Gate of Babylon contains countless noble phantasms. But in the Naruto world, characters like Naruto in his Six Paths Sage Mode are also extremely powerful. He has the power of the tailed beasts and can use all kinds of powerful jutsu. However, if we consider a crossover, it might be someone who can combine the powers from both worlds. For example, if a mage from Fate could learn to use chakra and combine it with their magic, they could be very powerful. Or if Naruto could somehow gain the abilities of a Servant, like the ability to use noble phantasms, he would be unstoppable.
One popular 'Fate Stay Night Naruto Crossover Fanfic' is 'Naruto: The Fifth Holy Grail War'. It combines the unique abilities and characters from both universes in an interesting way. The story often has Naruto characters participating in the Grail War, with some amazing battles and unexpected alliances.
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 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.
The combination of different power systems. In Naruto, there are ninjutsu, taijutsu, and genjutsu, while in Fate Stay Night, there are the powers of the Servants and the magic of the mages. When these two collide in a crossover fanfic, it creates a whole new set of possibilities for battles and character interactions.
Well, one big challenge is combining the different power systems. In Fate Stay Night, you have magic and Servant abilities, while in Halo, it's all about advanced technology and physical combat skills. Figuring out how to make these two coexist in a believable way is tough.
There's also 'Fate in the Halo Universe'. This one is about how the Fate characters find themselves in the Halo setting. It explores how they interact with the UNSC and the various alien species. For example, Saber's honor code clashes and also meshes in interesting ways with the military values of the Spartans.
One common theme is the clash of power systems. Since Bleach has its spiritual powers and zanpakuto, and Fate Stay Night has magic and Servants, seeing how these interact is a big draw. For example, how would a Servant fare against a Hollow in Bleach? Another theme is the exploration of different ideologies. The characters in both series have distinct beliefs and values, and fanfictions often explore what happens when these collide.
A cool 'fate stay night crossover fanfiction' idea might be a crossover with 'Final Fantasy'. The different classes and magic systems in Final Fantasy could blend well with the Fate universe. For example, a character from Fate could learn the job classes from Final Fantasy. Also, a crossover with 'Persona' could be interesting. The Velvet Room and the concept of personas could be integrated into the Fate world, adding a new layer of mystery and power to the story.
One great rwby fate stay night crossover fanfiction is 'The Convergence of Destinies'. It weaves the worlds of RWBY and Fate Stay Night together really well, with exciting battles and interesting character interactions.