- There are perfectly good reasons why these specific riders rejoin the Rider War while the rest didn't.
- Ryuki: Shinji's the Nice Guy. Even after Ore Journal closes down, he still wants to do good in the world hence why Sara would want him to come back. It's also a HUGE bonus that he is or once was Kamen Rider Ryuki which means that he'll have the most experience to go up against Sara's murderous boyfriend.
- Knight: Ren's participation is due to being the winner of the Rider War. Since he instinctively protects Shinji from Ouja because of a feeling he has, there is some merit to it.
- Ouja: Asakura's the Blood Knight. He wants to fight, short and simple.
- Zolda: Kitaoka has already passed away because of his condition. How Goro returns as Zolda remains a mystery until you remember in the final moments of the episode, Asakura was already out from prison. It might be possible Asakura got his payback on Kitaoka and Goro might have witnessed his boss's death.
- Raia: Since Raia is like a seer, he will most likely pick up on what is happening and would be drawn to the new war.
- Gai: Jun wanted a new challenge and the Rider War is the perfect opportunity for it.
- Ryuga: As revealed in Zi-O's Ryuki arc, as long as the real Shinji is still alive, Dark Shinji will exist in one way or another. Thus, when Shinji rejoins the Rider War, it's no surprise that Dark Shinji rejoins as well.
- Scissors: Sooner or later, Sudo's crimes will be brought into light. He could have either fled from the country or was arrested and later executed.
- Verde: Takamizawa did ran a successful (if shady) business, but since this world is a world where all Kamen Rider exists (san Decade), he could have either been arrested by some certain law enforcers who caught wind of his shady business dealings or have other major rival companies took over his, leaving him broke which when you think about it, sounds just right.
- Imperer: The whole reason Mitsuru got involved was because he wanted to make money easily by hiring himself to other Riders, but since his father's death wasn't caused by the Rider War, he still became the boss of his father's company and as a result he has no reason to participate since he already has everything he wants.
- Tiger: Tojo only got involved in the Rider War because his mentor, Hideyuki Kagawa aka Alternative Zero, by sheer chance had managed to read one of Shiro's research on the Mirror World. Because the Mirror World no longer exists in this timeline, Kagawa would never have learned about that world and thus preventing Tojo from participating.
- Femme: If we take into account Miho's backstory, it's possible this version of Miho might be the same. She tried to get revenge on Asakura but since Asakura's noticeably stronger this time around (being capable of fighting against Ryuga equally when his original self couldn't), Miho might've got herself killed by Asakura this time around before the Rider War can be restarted...
Fridge / Kamen Rider Zi O Rider Time Ryuki