A possible plot for this crossover fanfiction could be centered around the concept of the 'ideal'. In Fate Stay, each Servant has an ideal they fight for, and in RWBY, the characters also have their own beliefs and goals. So, a story could be crafted where the characters from both universes have to compare and contrast their ideals. Maybe some of the Fate Stay characters are confused by the concept of the Grimm and the Huntsmen/Huntresses' role in RWBY, while the RWBY characters are intrigued by the Servant - Master relationship in Fate Stay.
Another plot could involve the heroes from Fate Stay Night being transported to the world of the Shield Hero. They then have to adapt to the different rules of that world, like the wave system. For example, Saber could try to fight the waves using her Excalibur, but find that it's not as effective as she thought due to the different magic systems.
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 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.
One idea could be to have the characters from RWBY enter the Fate universe and participate in a Holy Grail War. Ruby could be a Saber - class servant with her scythe as her weapon. Another idea is to have the Fate characters visit Remnant and help the RWBY team fight the Grimm. For example, Archer could use his projection magic to create weapons for the RWBY team.
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 idea could be that the Servants from Fate end up in the RWBY world and have to team up with the RWBY characters to fight a new common enemy. For example, a powerful Grimm that has some sort of magic ability similar to a Servant's Noble Phantasm. Another plot could involve the RWBY characters being summoned into the Fate universe and having to participate in the Holy Grail War, but with their own unique skills and abilities that disrupt the normal flow of the war. A third idea might be that a mysterious force starts fusing the two worlds together, and the characters from both have to figure out how to stop it while also dealing with the cultural and power differences between them.
Perhaps Shirou from Fate Stay Night could find himself in the Re Zero world. He might use his skills as a mage and his determination to help Subaru in his quests. Their different outlooks on life and combat could lead to some interesting character development as they learn from each other.
The most interesting part is the combination of different powers. In RWBY, we have the unique Semblances and in Fate Stay Night, there are powerful Servant abilities. When these are combined in a crossover fanfiction, it creates really exciting battle scenarios.
One idea could be a crossover with the Justice League. Dr Fate could use his magic to help the League against a powerful magical threat that they can't handle alone. Maybe a dark sorcerer from another dimension is trying to invade Earth, and Dr Fate's knowledge of ancient magic is crucial in devising a counter - attack.
One idea could be that the Servants from Fate are transported to the Halo universe. For example, Saber could find herself in a battle against the Covenant. Her noble phantasm could be a powerful weapon against the alien foes. Another plot could involve Master - Spartan team - pairings. A Spartan could be paired with a mage to fight a common enemy that threatens both the Halo and Fate worlds.