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.
How about Gai Amatsu from Kamen Rider Build? He is a complex character with his own motives. His presence could create some interesting power plays and plot twists in the crossover fanfiction. He might initially try to use Kamen Rider Drive for his own gain but then end up having a change of heart.
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.
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.
Characters from Kamen Rider Decade are also frequently included. Decade has the ability to travel between different Kamen Rider worlds, so it makes sense for him to be in a 555 crossover. His presence can introduce new story elements and bring different Kamen Rider series together in a single fanfiction.
Definitely Kamen Rider Decade is a very popular character to include. His ability to transform into various Kamen Riders gives endless possibilities for crossovers. And then there's Kamen Rider Zi - O, as the time - traveling Rider, he can bring a whole new dimension to the kamen rider w crossover fanfiction. Additionally, Akiko Narumi from the Kamen Rider W series herself is often included, as she can add a touch of normal - human - in - a - Rider - world perspective to the story.
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.
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.
Tsukuyomi is also quite popular. She has her own set of abilities and a connection to Zi - O. In crossovers, she can interact with other female characters from different Kamen Rider series in unique ways. Also, Kamen Rider Decade is often included. Since he has the ability to travel through different Rider worlds, he can bring a lot of chaos and new story opportunities to a Zi - O crossover fanfiction.
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.
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.
Super Sentai characters would be great. For instance, a member of a Super Sentai team could team up with Zero One. They could exchange fighting techniques and learn from each other. The Sentai member might teach Zero One about teamwork in a large group, while Zero One could show the Sentai member some advanced technology - based attacks.