Firstly, I wouldn't touch the worlds of Kuuga, Agito, Hibiki, Kabuto and Kiva. Those stories are actually well done.
The idea of Ryuki's world being one big courtroom is just fail waiting to happen. Natsumi was right to demand an actual lawyer, even though she only got Tsukasa.
Here's how I'd have written the world of Ryuki:
Story of the Riders:
Each of the 12 riders are terminally ill. The Mirror World is a limbo, where they can keep their bodies whole. If the rider returns to the real world, the clock starts counting down again. The person who created the Mirror World comes up with a panacea that can cure the diseases. But because he's a sadist, he decides to make them fight for it. If they want the cure, they have to prove themselves worthy.
How the Hikari Crew Gets Involved:
Yuusuke witnesses Knight leaving the mirror realm. He brings him to the photo studio where Natsumi and Eijiro care for him.
Ryuki and several others are dispatched by the sadist Doctor Shiro Kanzaki to hunt down Knight. This leads into the first rider on rider fight. Kuuga, Knight and Decade on one side, Imperer, Odin and Ryuki on the other side. In the fight, Decade obliterates Imperer, while Kuuga Titan injures Odin.
Knight and Ryuki fight, diving back into the mirror world. Once here, however, Knight drops his weapon, telling Ryuki to finish him, because he's discovered that Kanzaki isn't going to give anyone the cure.
Decade enters, intercepting Ryuki's Sword.
Decade's Speech:
"Is your life worth so much to you that will kill another man to keep it? Life itself is a precious gift. Even if your life is soon to end, you still have the right to live it as you wish."
The Fight:
Four Riders: Decade, Kuuga, Ryuki and Knight find Kanzaki, who reveals himself to be Odin. Odin uses the Unite Vent to combine GoldPhoenix with Genocider. Knight and Kuuga fight Odin, while Decade's Ridebooker spits out the Ryuki cards. Final Form Ride to destroy the monster, then Odin attacks Knight. Ryuki goes dragon form again to save him.
In this moment, when Ryuki chooses to give his life to save another, Ryuki and Knight get a second wind, rising above their injuries. Decade uses the Illusion card to create duplicates to hold down Odin, so Ryuki and Knight can hit him with their finishers at the same time.
Decade's Photograph
The main image shows the riders receiving the panacea cure. The overexposure shows Ryuki and Knight shaking hands.