For a 'Kamen Rider crossover feudal Japan' fanfic, a Kamen Rider would be a must - have main character. In addition, a ronin, a masterless samurai, could be another main character. This ronin could have his own code of honor and be very skilled in combat. He and the Kamen Rider might meet in a dangerous situation, like being chased by bandits. And there could also be a young princess who is caught in the middle of a political intrigue in the feudal court. The Rider and the ronin might end up protecting her and in the process, get involved in the larger events of feudal Japan.
Well, it could be about a Kamen Rider being transported to feudal Japan. Maybe he has to fight against some ancient evil that has been awakened in that era. He could use his modern Kamen Rider powers in the midst of samurais and shoguns, creating a really interesting contrast between the high - tech and the traditional.
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.
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.
A powerful and corrupt daimyo could be the main antagonist. He rules with an iron fist, taxing the peasants heavily and suppressing any form of dissent. He sees Naruto as a threat to his power, especially when Naruto starts to gain a following among the oppressed people. He uses his samurai army to try and stop Naruto from spreading the ideas of freedom and equality.
Well, perhaps the fanfic could revolve around Kickhopper finding himself in an alternate universe where the rules of being a Kamen Rider are different. He has to adapt and form a crossover alliance with a local hero. They start by comparing their transformation devices and learn from each other's fighting styles to defeat a group of interdimensional thieves who are stealing important artifacts from both universes.
Definitely other Kamen Riders like Kamen Rider Kabuto. Their different powers and personalities would make for an exciting crossover. They could team up with Faiz to fight a common enemy or have a friendly rivalry.
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.
It could be Orochimaru from the Naruto universe. Imagine he finds a way to cross over to the Kamen Rider world. He would be attracted by the unique powers there and want to use them for his own evil experiments. His ability to manipulate and deceive would make him a formidable villain against both Naruto and the Kamen Riders.
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.