A powerful, ancient evil force that has been awakened by Dark Decade's actions could also serve as an antagonist. This force might be something that the Kamen Riders have never faced before and Dark Decade may have to either fight it or join forces with the other Riders to defeat it.
Another antagonist could be a group of Kamen Riders from different timelines who see Dark Decade as a threat to the stability of the multiverse. They might band together to stop him, even if he has good intentions in the fanfiction's plot.
Another Kamen Rider from the same series who has a different view on justice could be an ally. They might see that Cronus's power could be used for good if guided in the right direction.
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.
One idea could be exploring the backstory of Dark Decade more in - depth. Maybe there was a tragic event in his past that led him to his current path. Another could be a crossover with other Kamen Rider series, where he teams up with a Rider from a different timeline.
Well, Sonic the Hedgehog could be an unexpected character. Imagine Decade and Sonic teaming up to outrun an enemy that can move at super - speed too. Another one could be Pikachu. Decade might have to deal with Pikachu's electric attacks while they fight a villain together. And how about Harry Potter? Decade could end up in the wizarding world and team up with Harry to defeat a dark wizard who has somehow got hold of Kamen Rider technology.
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 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.
In a crossover with Kamen Rider Decade, Decade could be a potential ally. Decade has the ability to travel through different Kamen Rider worlds and could help Cronus if they find themselves in a complex multi - world situation. Also, Kamen Rider Kuuga might be an ally. Kuuga has a strong sense of justice and his raw power could complement Cronus' more calculated fighting style.
In terms of anime characters, Ichigo Kurosaki from 'Bleach' could be an interesting addition. His ability to see and fight spirits could be related to Kiva's battles with Fangires. They could end up in each other's worlds and have to learn to use their powers together to fight off both Hollows and Fangires.
Another possibility is Kamen Rider Decade. Decade has the ability to transform into different Kamen Riders, and his presence in a crossover with Kickhopper could bring a lot of different elements into the story. He could use his powers to help Kickhopper access new forms or abilities, or they could travel through different Kamen Rider worlds together to stop a multiverse - level threat.