It was inevitable, really; to assume that the Rider of Destruction and Creation would be the only one to wander the endless parallel worlds of the multiverse was a foolish notion. Eventually, he stumbled across another Kamen Rider akin to himself, the Rider of Chaos and Change, Kamen Rider Remnant. Both were eternal wanders, forever passing through the worlds of countless other Kamen Riders, forever altering and changing them in their passing. Eventually, they had struck up a strong friendship, formed by their similar fates and further tempered by travelling together and traversing many challenges and trials side-by-side. Together, they heralded chaos and destruction in their arrival; but brought forth change and creation in their passing.
It was Remnant who eventually proposed the idea to Decade, one day on their travels. "You were the leader of Dai Shocker before, weren't you?"
To which, Tsukasa Kadoya; Kamen Rider Decade, responded in confusion, "Um...yes? But I'm not anymore; those days are behind me - and no the whole war against the Super Sentai doesn't count! That was me going undercover to-"
"Yeah, yeah, yeah; I've heard, I've heard," Remnant waved, cutting him off, "But; that means you know how to lead a huge, massive, multiversal organization of really powerful and eccentric people, right?"
"I...suppose I do, yes." Tuskasa answered. "Why?"
"Well, I was thinking..." Remnant trailed off innocently yet full of mischief, "If you have the moxie and know-how to lead a massive multiversial organization dedicated to evil, and now that you're a good guy who still has those skills; who also knows a whole legion of really powerful and eccentric good guys...why not do what you used to do before, only the opposite?"
"I'm sorry, what?" Tsukasa asked, confused by what Remnant was asking. "I don't get what you're asking here..."
"It's simple, silly!" Remnant exclaimed, twirling around and pointing boldly at the Rider of Destruction, "You formed Dai Shocker; a multiversal organization devoted to evil and world domination when you were a bad guy. Now I'm asking why you haven't gotten around to forming a multiversal organization dedicated to good and world salvation, uniting all the Kamen Riders you've met in all of your travels! You can even call it Dai Rider-no, the Dai Rider Zaibatsu! That sounds way cooler."
"I-" Tsukasa actually stopped, mulling the question over, before finally answering in a baffled tone, "I...don't know why I never considered that. I guess, maybe I just wanted to let the other Riders live their lives in peace?"
Remnant scoffed, then retorted back, "Oh please, as if any of them would actually say no to continuing to fight for justice; being a Rider is a calling, a way of life even. And once a Rider, always a Rider. Not to mention how it could help everybody out.
Think about it, the sheer number of Riders standing side-by-side; the multitude of powers working together in unison! All those Kamen Riders who were condemned to tragic fates at the end could be saved if the right Riders where there beside them to help. New Riders could learn from the older, more experienced Riders; reaching their full potential and having someone to call on in times of true peril! Think of all the tragedies so many Riders go through, and how something like the Dai Rider Zaibatsu could've prevented them all!
Not to mention, I'm pretty sure Dai Shocker is still mucking about somewhere in the multiverse; they're like fucking cockroaches..."
Tsukasa took in the revolution idea of his traveling partner, and thought it over. "...the Dai Rider Zaibatsu, hm?"
And so it came to pass; the magnum opus of Kamen Riders Decade and Remnant, a multiversal organization formed from all the Kamen Riders in the known multiverse, united under a single banner: the organization known as the Dai Rider Zaibatsu. Every day, more and more Riders are scouted out and invited to this prestigious faction; a united front against the forces of evil like Dai Shocker and others like it.
And as this organization grew, so too did the renown of Kamen Riders across the multiverse grew. Soon, all of reality knew this truth to be self-evident; that even if God or Buddha may be dead, even if all hope was lost and there was naught but despair across the horizon...
There would always be...Kamen Rider...
And so, it was with great pride that you held yourself up high as a member of this illustrious organization; yourself a Kamen Rider, a warrior and soldier of justice and salvation. You had fought alongside these fellow heroes, bled besides them in the field of battle and aided them in their times of need; just as they had you.
From the older Showa generations who mentored you, to the Heisei generations who fought evil out in all the worlds day by day, to even the newest of Riders who were newly recurited to the zaibatsu; these men and women were your comrades, your brothers and sisters in arms, you...familiy even.
But, enough reminiscing; you had more important things to tend to as...
[X] An elite Rider; one of the best of the best in the Dai Rider Zaibatsu, you were held in the same regard as the likes of Kabuto and Kuuga. Whereas your average Rider was sent in to fend of armies of evil to save a world, you were deployed against armadas spanning solar systems and fought horrors that devoured said systems as snacks. It's a tough job, but you were up to the task. GM's favorite and most fleshed out of the ideas
[X] A rookie Rider; newly enlisted and picked up from his home reality, you were just starting out and more on par with Den-O or Ryuki. However, you knew that you were just starting to grow into your potential, and the Zaibatsu was more than willing to help you; especially with the old guard there for you to learn from. Fleshed out so-so by the GM
[X] A veteran Rider; past his prime and more suited to passing the torch to the newer generation, though you still had some kick like the first Kamen Rider did. Though you spend most of your time teaching the new blood the tricks of the trade, you still went on missions yourself; you weren't that old, not yet anyways. GM's gonna have to do a lot of rewatching for this one...
Wise Up!
Kamen Rider Remnant - If Decade can be consider the Rider of Destruction, who also brought Creation in his wake; then Remnant is the Rider of Chaos, who in turn brought necessary change in their wake. Their powers are also similar to Decade's; in addition to travelling across the multitude of realities, Remnant's transformation belt drew on the 'Possibilities of Riders Yet to Come'. Unlike Decade, who can draw upon and change into any of the other Riders in existence past or present; Remnant turns into the Riders who will exist in the future or may have existed had the past not turned out the way it did.
Co-leader of the Dai Rider Zaibatsu alongside Decade, Remnant is considered one of the most powerful Riders in existence just as Decade is, for the exact same reason: the sheer staggering potential and versatility of their powers. In fact, the Final Form Ride of Remnant and Decade is considered the strongest Rider to ever exist; encapsulating the very essence of Kamen Rider - by drawing upon the power of all Riders past, present, future, and what may have been.
So...I got bored again and came up with this in a fever dream; because why not?