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.
There could be a fanfiction where the characters from Kamen Rider Ex - Aid end up in a parallel world. In this world, the rules of the games and the powers of the Bugsters are completely different. They have to figure out how to adapt and defeat the new threats. For example, the healing items might work in reverse, so they need to find a way around it.
Definitely Kamen Rider Decade. His ability to travel between different Rider worlds would make for an exciting addition. He could bring in elements from other Rider universes and create some really unique interactions with the Ex - Aid characters. For example, his different forms could be used in unexpected ways to combat the Ex - Aid villains.
One interesting crossover could be the use of Naruto's ninja skills in the world of Kamen Rider Ex - Aid. For example, Naruto's shadow clone jutsu could be used to confuse the Bugsters in the Ex - Aid universe. Another crossover might involve the Riders using chakra - like energy, inspired by Naruto's world. Maybe they could even learn some basic ninja moves to enhance their combat abilities.
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.
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.
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.
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.
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.
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.