Maybe a crossover with a non - Kamen Rider franchise, like a popular anime. Imagine Cronus finding himself in the world of an anime and having to adapt to their rules while also trying to find a way back to his own world. He could encounter characters from the anime who are intrigued by his powers and technology, and there could be a battle of ideologies between Cronus' more self - serving view and the anime characters' values.
One interesting plot idea could be a crossover with another Kamen Rider series where Cronus has to team up with a Rider from a different timeline. They could face a common enemy that threatens both their timelines. For example, a powerful time - traveling villain who wants to rewrite history to make himself the supreme ruler of all timelines. This would force Cronus to set aside his usual self - centered ways and work with the other Rider.
A plot could involve Cronus crossing over into a parallel world where he is the only Kamen Rider, but there are new and strange threats. He has to figure out how to use his powers in this different environment. There could be a mystery surrounding why he ended up in this parallel world and who or what is responsible. Along the way, he might discover new forms or abilities as he battles against the unknown forces.
Another plot could involve Cronus teaming up with a former enemy to face a greater threat. This would force him to re - evaluate his relationships and motives. For example, he might have to work with a Kamen Rider he once tried to destroy. It could lead to some great character development for him.
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.
A crossover with Kamen Rider Zi - O could also be really cool. Zi - O travels through time, so he could accidentally disrupt the timeline in a way that affects Ghost's world. Ghost might have to team up with Zi - O to fix the timeline while also dealing with the enemies that are taking advantage of the time - chaos. They could encounter different versions of themselves from alternate timelines and have to figure out how to make things right.
One interesting plot idea could be Chalice crossing over with another Kamen Rider series, like Kamen Rider Decade. They could team up to fight a common enemy that has the ability to travel across different Rider worlds. Maybe this enemy is stealing the powers of various Riders, and Chalice and Decade have to figure out how to stop it while also learning about each other's abilities and fighting styles.
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.
One idea could be a crossover with another Kamen Rider series like Kamen Rider W. The two Riders could team up to fight a new, powerful villain that is causing chaos across both of their worlds. They would combine their unique abilities and gadgets to take on the enemy.
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.
A really cool plot could be a time - travel crossover within the Kamen Rider Ex - Aid universe itself. The Riders could travel back in time to the early days of the Bugster virus outbreak. They would have the knowledge of how things turned out in the future, but would face challenges because changing the past could have unforeseen consequences. This would lead to a lot of moral dilemmas for the characters as they try to stop the virus in a different way without making things worse.
Another plot idea is a time - travel crossover. Zero One could be sent back in time to the era of the first Kamen Riders. There, he has to adapt to the different fighting styles and technology of that time. He might also introduce some of his own futuristic technology to the early Kamen Riders, changing the course of their battles and the development of their powers.
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.