Character interaction is a key element. For example, how Saber interacts with the characters from the other universe. If it's a crossover with a superhero universe, seeing her exchange ideas or fight alongside them is important.
A plot could revolve around a spell gone wrong that merges the two worlds partially. Saber finds herself in a strange new world filled with devils, angels, and fallen angels. She might initially be confused but quickly adapt. Her presence could disrupt the balance of power in Highschool DxD. For example, the devils might see her as a threat or an opportunity to gain more power. And Saber would have to navigate this new world while trying to find a way back to her own world.
Well, if Naruto summons Saber, it could lead to some really interesting battles. Saber's combat skills and noble phantasms could be a great addition to Naruto's world. She might face off against powerful ninja like Sasuke or Pain. Maybe she could help Naruto in his fight against the Akatsuki. It would be a very different dynamic compared to the normal ninja battles in the Naruto universe.
There's also 'Saber Meets the Avengers'. This fanfiction has Saber somehow crossing over into the Marvel universe. It's really interesting to see how she would handle battles alongside superheroes like Iron Man and Captain America. It usually has a lot of action-packed scenes.
Saber could fit in as a powerful ally. Her noble phantasms and combat abilities would make her a force to be reckoned with. In a world full of supernatural beings like Highschool DxD, she would be another strong addition. She might be seen as a sort of mysterious figure that both the devils and the angels would be curious about.
Well, a fanfiction like this could be really interesting. Maybe it could start with Rin and Shirou being somehow transported to the DxD universe. Saber could end up being a powerful ally or a mysterious figure that both the devils and the heroes in the DxD world are curious about. And Astaroth, being a powerful demon in the DxD lore, would surely have an interesting dynamic with the Fate characters. It could be a story full of magic battles, political intrigue within the devil society, and the clash of different ideologies from Fate and DxD.
One way the fanfiction expands on the characters is through their interaction with new elements. Shirou, who was mainly dealing with his own world's magic system, now has to deal with Saber's different magical heritage. This forces him to adapt and learn, showing his intellectual side more. Saber, being from a different time and place, in a crossover might have to face modern - day ethics and values. This challenges her traditional beliefs and shows how she can either hold on to them or evolve. Their relationship in the crossover also allows for exploring their mutual respect and how they can support each other in ways that were not as evident in the original story.
One unique feature is the combination of different universes. For example, when Saber is crossed over with a modern - day setting, it creates a contrast between her medieval values and the contemporary world. This leads to interesting character development as she has to adapt.
Well, a great storyline might involve Shirou and Saber swapping roles in some way. For example, Saber having to take on Shirou's daily life tasks, like going to school or doing mundane jobs, while Shirou has to lead in battles like Saber usually does. This role - reversal can bring out a lot of comical and also deep character - development moments. Also, a plot where they are in a parallel universe where their fates were different from the original story and they have to come to terms with these new versions of themselves.
In this crossover, Shirou's projection magic could be incorporated by having him project not only weapons but also magical items from the DxD universe. This would show his adaptability. Rin could combine her magic with the devil - related magics in DxD, creating new spells. Saber's noble phantasm could be the key to solving certain major problems in the DxD world, like a powerful threat that even the devils can't handle. Astaroth's powers could be used to test the limits of the Fate characters' abilities, forcing them to grow and develop new strategies to deal with him.