One idea could be that Sento and Banjou have to face a new threat that targets their friendship. Maybe an enemy uses mind - control to make them turn against each other, and they have to break free from it to save the city. Another could be a time - travel plot where they go back in time and meet their earlier selves, which causes all sorts of chaos in the timeline.
Well, a great plot for a fanfiction about them could be a mystery - solving one. A strange phenomenon is occurring in the city, and they have to use their combined knowledge and skills to figure out what's going on. Along the way, they might discover some long - lost secrets about the Kamen Rider Build technology. As they dig deeper, they find that the mystery is tied to their very existence as Kamen Riders. This forces them to re - evaluate their roles and what it means to be a hero.
A possible plot could involve Sento and Banjou getting trapped in an alternate dimension. In this dimension, all their powers work differently. They must figure out how to adapt and work together to find a way back home. Also, a story where they swap bodies would be really interesting. They would have to deal with each other's lives and responsibilities while trying to find a way to switch back.
Their relationship could be developed by having them go through a really tough trial together. For example, if one of them is severely injured and the other has to take care of him while still fighting the bad guys. This would show a deeper level of care and dependence.
One idea could be an OC who is a scientist from a different dimension and has a unique build system. For example, instead of using bottles, they use crystals with special powers. Another idea is an OC who is a street racer by day and a Kamen Rider by night, with their racing skills incorporated into their fighting style as Build. Or perhaps an OC who is a young prodigy in the field of robotics and creates their own Kamen Rider Build - inspired suit with advanced robotic functions.
One idea could be that the Avengers are transported to the world of Kamen Rider Build. They team up with Build to fight a common enemy that has the power to disrupt both universes. For example, a villain who can manipulate matter like Build's powers but on a much larger scale, threatening Earth in the Avengers' world and Build's city in his world.
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.
One idea could be a story where Kamen Rider Phantom has to team up with an unexpected ally from a different dimension. This ally could bring unique powers and a different perspective, creating new challenges and adventures for Phantom.
Another plot could involve Kamen Rider Gaim getting trapped in a time loop. He has to relive the same day over and over again, each time trying to figure out how to break the loop while also dealing with the day - to - day threats he usually faces. In the process, he discovers new things about his powers and the true nature of the threats in his world.
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.
Maybe there's a device in the Kamen Rider Drive world that can unlock hidden powers in Naruto. This power could be related to his chakra, but in a new and different way. Like his chakra could be used to power up Drive's abilities even more. And in return, Drive could teach Naruto some cool combat moves that are different from his ninja training.
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.