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.
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.
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 challenge is combining the two very different universes. Naruto's world is full of ninjas, chakra, and a feudal - like society. Kamen Rider Decade's world has high - tech elements and the concept of Riders. Merging these two concepts without making it seem forced can be difficult. Another challenge is staying true to the characters. Naruto is known for his determination and his goal to become Hokage, while Decade has his own mission to travel through different Rider worlds. Keeping their personalities intact while putting them in new situations is not easy.
One interesting Kamen Rider Decade fanfiction could be about Decade traveling to different Rider worlds and teaming up with alternate versions of himself. For example, he might meet a younger version of himself in the Kuuga world and they have to work together to defeat a new, powerful enemy that is threatening all Rider worlds.
Natsumi Hikari is a popular character in Kamen Rider Decade fanfiction. She provides a sort of 'normal' perspective in the otherwise extraordinary world of Riders. Writers might explore her growth from being a somewhat reluctant companion to Decade to becoming a more confident and important part of his adventures. Her relationships with other characters, especially Decade, are often a focus in fanfiction.
Tsukasa Kadoya, who is Kamen Rider Decade, is a key character. Then there are the other Kamen Riders from different worlds that he encounters. For example, Kamen Rider Kuuga. Their interactions with Decade shape the story. Also, Natsumi Hikari plays an important role as she is often involved in Decade's adventures and provides a different perspective compared to the more combat - oriented Decade.
One of the main storylines often involves the journey of the Kamen Rider Decade through different Rider worlds. He has to face various challenges and foes specific to each world he visits.
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.