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.
One unexpected character could be Snoopy. Picture Decade in the Peanuts world, with Snoopy as his sidekick, flying his doghouse to chase down a villain who has the ability to turn the Peanuts characters into stone. Then there's Sherlock Holmes. Decade could cross paths with Holmes in Victorian London. Holmes' deductive skills combined with Decade's fighting abilities would make for an interesting story as they solve a mystery related to Kamen Rider powers. Also, Goku from Dragon Ball Z. Decade and Goku could have an epic battle - cum - team - up against a powerful foe that threatens both their universes.
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.
Well, other Kamen Riders are very likely. For instance, Kamen Rider Chalice, as he has a connection with Blade in the original series. Their interactions in a crossover could explore more of their relationship and how they work together against new threats.
In a Kamen Rider Decade Crossover Fanfiction, Decade could end up in a world where the Kamen Riders are all children. He has to protect them from an evil force that wants to use their power before they fully understand it. This would involve him teaching them how to use their Rider powers in a more creative and simple way. He could also cross over with a classic monster movie world, like Godzilla's world. Decade and the Kamen Riders have to work together with the monsters to stop an even greater threat.
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 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.
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.
Kamen Rider Decade is often a popular choice. His ability to transform into other Riders makes him a natural for crossovers. He can bring in elements from different series and connect various storylines together.
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.
Iron Man would also be an interesting crossover character. With his high - tech suit, he could team up with Kamen Rider Mach. They could share technology and upgrade each other's gear. Imagine them combining their firepower to take on a huge mecha - like enemy. Iron Man could provide aerial support while Mach attacks from the ground.
Kamen Rider Decade would be an excellent choice. Since Decade can travel between different Kamen Rider worlds, it would be easy to create a story where he and Orga meet and team up. Another good option is Kamen Rider Kuuga. Their different fighting styles and powers could create an interesting dynamic. Also, Ultraman could be a great crossover character. The contrast between the Kamen Rider and Ultraman universes would make for a very exciting story.