Well, perhaps in this crossover fanfiction, the power systems of both could merge. For example, the magical abilities of the Fate Stay Night characters could combine with the Freezing characters' abilities. Let's say Saber's Excalibur could be enhanced by the energy manipulation skills of a Freezing character. And there could be a love triangle between a Fate Stay Night male character, a Freezing female character, and another character from either universe, which adds a lot of drama to the story.
In a Fate Stay Night Skyrim crossover, Archer could be summoned by a powerful mage in Skyrim. Archer would find the concept of the Thu'um fascinating as it's a form of magic he's never encountered. He could start exploring Skyrim, looking for ways to use his own abilities in conjunction with the local magic. He might get involved in the civil war between the Stormcloaks and the Imperials. His strategic mind could help whichever side he decides to side with. And when the dragons return, he could use his Unlimited Blade Works to create weapons specifically designed to fight dragons, like blades that can pierce dragon scales more easily.
Another great combination could be Archer from Fate Stay Night and Kazuya from Freezing. Archer's cynical and experienced nature would contrast well with Kazuya's more naive and kind - hearted personality. In a battle situation, Archer's long - range archery skills combined with Kazuya's support abilities for the Pandora could create some really cool combat scenes. Plus, their different outlooks on life would lead to some interesting philosophical discussions between them.
A great plot could involve the magic systems of both universes colliding. For instance, the spells in Harry Potter and the magecraft in Fate Stay Night. The Grail in Fate Stay Night could be related to a powerful and dark magical artifact in the Harry Potter world. Characters like Shirou could meet Hermione, and they could compare their knowledge of magic. Shirou might learn some new spells from Hermione, while Hermione could be intrigued by Shirou's ability to reinforce objects. This could lead to a joint effort to stop a common threat that is using the combined power of both magic systems for evil.
Perhaps a plot where the Holy Grail War from Fate Stay Night gets disrupted by the arrival of some key players from Fate Apocrypha. This could lead to new alliances and rivalries. Like Shirou from Fate Stay Night having to team up with a character from Fate Apocrypha, say Sieg, to face a common enemy that is after the power of the Grail in both their worlds.
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.
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.
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.
One possible interesting plot could be a character who has the power of mana suddenly gaining the ability related to od. For example, Shirou could discover that his mana can be combined with od in a unique way that gives him new abilities to face the challenges in the Holy Grail War. This could change the power dynamics and his relationships with other characters like Saber.
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.