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.
Another great option could be Naruto. He has his ninja skills and chakra powers. He and Kamen Rider Mach could cross paths in a dimension where both their worlds are merging. Naruto could use his shadow clone jutsu to create distractions while Mach zooms in for the attacks. And they could both learn about the different forms of power they possess.
A really cool plot could involve Kamen Rider Mach crossing over with a time - traveling hero. They have to go back in time to stop an evil organization from changing history in a way that would be disastrous for the future. There could be lots of cool chase scenes and battles in different historical eras.
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.
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.
Definitely Medic from Kamen Rider Ex - Aid. Her medical knowledge and combat skills could add an interesting dimension to the story. She could help Kamen Rider Drive when he gets injured during a battle in the crossover scenario.
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.
An interesting antagonist for a crossover could also be an evil Kamen Rider from a parallel universe. Let's say there's a version of a well - known Kamen Rider who has turned to the dark side. This evil Rider could be jealous of Chalice's powers and wants to take them for himself. He could be forming alliances with other villains from different Kamen Rider series in order to achieve his goal, making him a formidable and complex antagonist in the crossover story.
Takeru Tenkuji (Kamen Rider Ghost) should definitely be included as the main character from his own series. For a crossover with Kamen Rider Decade, Decade would be a great addition. His ability to transform into different Kamen Riders and his experience traveling through different worlds would bring a lot to the story. He could help Ghost navigate through different dimensions or face enemies that are familiar to Decade but new to Ghost.
In my opinion, Ultraman could be a great character to crossover with Kamen Rider Agito. They both have their own unique powers and fighting styles. Ultraman's large - scale battles against giant monsters could contrast with Agito's more street - level combat against the Unknown. And they could combine their powers in some creative ways, like Agito using his form - changing abilities to adapt to Ultraman's size when they need to fight a really huge enemy together.
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.