A more complex idea could involve the Mirror World from Ryuki. In the fanfiction, the RWBY characters could be accidentally pulled into the Mirror World. There, they meet the Riders from Ryuki and have to figure out a way to escape. Along the way, they learn about the rules and dangers of the Mirror World, and also discover some secrets about their own powers.
You could start by mixing the character traits. For instance, take Weiss's elegance and combine it with Ryuki's determination. Maybe Weiss gets a Kamen Rider - style power - up that is related to her Dust - using abilities. This would create a unique dynamic.
Well, from the Kamen Rider universe itself, characters from Kamen Rider Blade could be great for a crossover. Their different powers and personalities would create interesting interactions with the Ryuki characters. Another suitable character could be Kiva. His vampire - like nature and unique fighting style would add a new flavor to the Ryuki crossover.
One idea could be a crossover with other Kamen Rider series. For example, Ryuki characters interacting with those from Kamen Rider Decade. They could team up to fight a common enemy that has the ability to travel between different Rider worlds. Another idea is a crossover with a popular anime. Maybe Ryuki riders in the world of Naruto, where they have to adapt their fighting styles to the ninja world.
Ryuki is often very popular. He's the main character in the original series, so he's a natural focus in fanfiction. His journey and his growth as a rider make him an interesting character to write about.
One idea could be exploring an alternate universe where the riders form different alliances. For example, some of the more antagonistic riders team up against a common threat that is far more powerful than any single rider. This could lead to interesting character development as they have to put aside their differences.
Ryuki himself is a popular choice. His complex backstory and his quest for justice in his own universe make him an interesting character to explore in a crossover. He has a strong sense of right and wrong and his fighting style is cool.
Another plot could involve a misunderstanding between the Kamen Rider Ryuki and Dragon Knight characters that leads to an initial battle between them. But as they fight, they start to realize they have more in common than they thought and end up joining forces. For example, they could discover that a shadowy organization is manipulating events to keep them at odds.
One interesting fanfic could be where the Kamen Riders from Build end up in the RWBY universe and team up with the huntsmen and huntresses to fight a common enemy. For example, they might face off against a Grimm that has been enhanced by some Evolt - like technology.
Well, Ruby might be really excited. She loves all things new and heroic, so she'd probably be into the cool action and unique characters in the Kamen Rider fanfiction. Weiss might be a bit more critical at first, but could end up enjoying the complex storylines. Blake would be interested in the deeper themes and character relationships, and Yang would just love the intense fight scenes.