I did this quite some time ago, but I still really like the idea, soooo...
[hide]
I would write it as all the Riders from the various worlds have been taken away to another world by an unseen force, leaving most of the worlds at the mercy of the monsters that inhabit them. With each world they visit, Tsukasa would come in contact with a person (or persons) he sees fit of being a Kamen Rider in that world and once they have some big revelation for whatever the story of the arc is, his Kamen Rider cards would not only unblank, but allow him to give a belt to whoever he sees fit. Though little by little it's revealed that by giving these other people Rider powers, Tsukasa has been throwing the worlds out of alignment and causing them to merge.
Yuusuke wouldn't travel with them and Daiki would be a much more active character. In the past, Daiki was a refugee from his own world and landed in a world where he meets someone using the Ace suit who he comes to think of as a brother. In this world, he discovers Dai-Shocker is a huge threat and planning to merge all of the worlds, he learns that they've made a counterpart to Decade, DiEnd, and takes it upon himself to steal it. As he's doing this, he meets Narutaki, who tells him of treasures in each world that Dai-Shocker will harness energy from, thus he goes to each world looking to keep these out of Tsukasa's and Dai-Shocker's hands. Narutaki and Daiki both know who Tsukasa is, but the viewer wouldn't learn the real story until the final episodes.
Episode break down
Natsumi's world - 1
Kuuga's world - 2~5
The crew arrives in a world being overtaken by the Grongi, Tsukasa meets a young police recruit who was a friend of Godai, his name is Odagiri. Odagiri senses something odd about Tsukasa and tails him throughout the episodes, eventually risking his life to save Yashiro, who has been poisoned.
Kabuto's world - 5~7
Tsukasa comes to this world and sees that ZECT is building new Kamen Rider suits, so he joins ZECT until that can be finished. He comes across a man named Tenjin, a young janitor who is incredibly shy, but willing to do anything for his friends. Throughout the episodes, ZECT would make various gears and eventually a Kabuto2.0 which backfires and nearly kills Tenjin, who still tries to use it to help the other Riders, and that's when Tsukasa sees him fit to be Kabuto.
Narutaki's antics: He tries to put three other Kamen Riders against Tsukasa, telling them that they will be able to save the world if they kill Tsukasa.
Ryuki's world - 8~12
Kanzaki is trying to create new Rider decks after the disappearance of chosen 13, to do this, he has snuck back into his college to work on the decks. Tsukasa is given the role of a new teacher in this world and hears rumors that have been popular with students about Kanzaki's ghost flying around school. He seems a little odd, but a photography student, Mamoru, takes a liking to him and follows him around one night where the two discover two new Riders Kanzaki has created, Abyss and Blue, who start using their school as the battleground. Tsukasa and Mamoru try to discover who these two really are and Mamoru is able to snap photos of them, nearly getting himself killed and then given Ryuki powers. Daiki shows up for the first time here, kills Abyss, and steals Kanzaki's research on Kamen Riders and other worlds.
Den-O's world - 13~16
The Imagin are running wild, attacking humans and causing a mess in both the past and present, but five renegade Imagin are there to help out the humans, who see all Imagin as a threat. Tsukasa encounters them all as he becomes a passenger on Den-Liner, after seeing how selfless they can be, traveling to the future and saving a young Kai, he gives the Imagin, Deneb, Momotaros, Ryutaros, Urataros, and Kintaros, the ability to become Den-O and Zeronos on their own. Humans still hate the Taros at the end of this arc, but they continue fighting to protect them.
Narutaki's antics: Humans are given ZECTrooper gear to fight the Imagin and try to kill off Deneb, the Taros, and Tsukasa.
Kiva's world - 17~22
The Fangire have forgotten about the humans and are more focused on taking the throne now that Wataru and Taiga are gone. Battles are raging all over the city, innocent people being hurt, and Tsukasa is cast as the judge in these battles. Kaiba, a man in his late 20's realizes that the Fangire will only destroy themselves if they keep fighting and is trying to stop them from doing that. Narutaki starts rumors of a device that will feed the Fangire for an eternity if the life of one person is given up, Tsukasa. Kaiba finds the device and tries to give up his own life, but Daiki has stolen the core to the machine, Tsukasa gives Kaiba Kiva powers and then ends up in a battle with Daiki, who wants to stop his world screwing once and for all.
Agito's world - 23~26
Narutaki arrives in this world before Tsukasa and discovers a method to give people the ability to fight on their own, making their Agito powers awaken. When Tsukasa and the others arrive, they discover a city with nothing but various colored Agitos running around attacking the Lords. Later on, Tsukasa meets a couple of humans who avoided evolving and want humans back to the way they used to be, including a man named Taichi, whose father has become an Agito.
Tsukasa's world part 1 - 27~28
To tie into the movie, Tsukasa arrives in his own world, where is welcome as a hero by Dai-Shocker. Not knowing what's going on, Tsukasa takes a liking to be treated like a king until Daiki appears and once again takes him into battle, only to be met by a horde of monsters. Tsukasa and Daiki both fight the monsters off, Daiki questions Tsukasa's loyalty, he has no idea WTF is going on and then his sister shows up to remind him of his past.
Kamen Rider Decade the Movie: Heart of the Fallen Destroyer
Tsukasa is kidnapped by Shadowmoon and his sister, leaving Daiki to defend Natsumi from another monster attack. Narutaki shows up and we learn of his past here, Daiki berates him for messing with the worlds just as much as Tsukasa has. Daiki is the hero for a good portion of the movie, learning Tsukasa isn't totally evil, at least not his present self. Tsukasa is saved from Shadowmoon, who has merged with his sister, and the other Dai-Shocker heads, to become Super Shadowmoon, he seemingly kills Narutaki and then awesome battle and yaaay Dai-Shocker is gone.
Tsukasa's world part 2 - 29~32
...or not. Daiki decides to finally join Tsukasa's crew, they go back to Hikari studio and as the mural flashes to signal a new world, but the image is the same, they haven't gone to a new world at all. Natsumi passes out. As she's passed out, she dreams about the Rider war and wakes up scared to death of Tsukasa. Later on we learn that Dai-Shocker isn't gone and that Apollo Geist has been manipulating Natsumi, who holds the power to something even greater than Decade, Decade Complete.
Hibiki's world - 33~36
Tsukasa is enrolled in Takeshi as a teacher of the Emergency Oni Replacement Team, a team of highly skilled warriors who aren't Oni yet. The Makamou have started to rampage without any proper Oni, attacking both humans and themselves. It's said that at least one Oni must exist for the world to be in order. Tsukasa meets Asumu, who teaches Natsumi about the Oni way and unlocks a dream that shows her and Tsukasa in a rather close setting. Asumu later saves Natsumi and then he becomes the new Hibiki.
Diend's world - 37~39
Basically plays out the same as the original version, but the reveal at the end won't be so out of the corner that it makes no sense.
Faiz's world - 40~42
The group arrives in Faiz world, where much to their surprise, they see a Kamen Rider Faiz running around. The head to Smartbrain to figure out what's going on and discovers that Narutaki isn't dead and still believes Tsukasa to be the ruin of the world, and now declares Daiki to be his enemy as well. Tsukasa discovers that the Faiz running around here is actually an android (Kento Handa in the role) that is starting to go haywire. Daiki is killed in a battle trying to protect Natsumi, who passes out thanks to an explosion that shows her and Daiki together in a home as children. Tsukasa then finds Keitarou and eventually makes him Faiz. But before leaving the world, Daiki somehow shows up at Hikari Studio, alive and well, not sure of what's happened.
Blade's world - 43~46
Daiki starts to realize that he's become an Orphenoch in this world as he loses DiEnd Driver and suddenly transforms when a monster attacks him. Tsukasa is put in charge of hiring new Kamen Riders for this world that will use secondary suits made by BOARD (the Aces), but he ends up finding four guys to become the original Blade Riders.
Rider War world - 47~51
Picks up right as the crew head back to Hikari Studio. They see the image for the Rider War world appear and head outside to see the Hikari Studio in the middle of a valley. Explosions are heard and then they see IXA, Leangle, Todoroki, and Gills fighting each other, as well as other monsters. They set their sights on the crew and begin to attack them, but the crew is saved by the original Ryuki, Hibiki, and Kiva. We learn that all the title Riders each have a connection to the heart of their own world that allows them to be protected from any brainwashing. It's shown that the other Riders have been taken over by Dai-Shocker, leaving only nine Kamen Riders to fight in this world and this is when we see all of the original title Riders come back. The Riders' final mission is to stop Dai-Shocker by killing Apollo Geist. Along the way, we learn that Natsumi was/is Tsukasa's wife, Daiki and Natsumi are siblings and that there was a time when Tsukasa was a normal human, but he was brainwashed by Dai-Shocker.
Natsumi had her memories removed by Narutaki, who was actually a friend of hers at one point and wanted to save her the pain of seeing her husband become evil. Narutaki's grudge against Tsukasa here is portrayed more as him being someone who was a friend of the two and who followed Tsukasa into Dai-Shocker to make sure he didn't go too crazy. Narutaki and Daiki were the only ones to know the whole story through the journey and both wanted to stop Tsukasa to save Natsumi, though they both had different ideas of just how to do this. Daiki was captured by Shocker but managed to escape, he intended to jump to a different world with Tsukasa, but the two were sent to worlds far apart from each other.
Daiki eventually dies as he saves Natsumi once again. It's learned that Tsukasa has actually accelerated the speed of which the worlds are colliding by giving other people Rider powers. Tsukasa and the other Riders storm Dai-Shocker castle for one Big Ass Fightâ„¢ against the final villain of every world, with each Rider disappearing as their worlds merge, leaving only Apollo Geist and Tsukasa in the end.
Conclusion: The worlds have all become one and Tsukasa is the last Kamen Rider left alive. New monsters appear and he begins to fight them and once again begins his search for new Kamen Riders who will protect this new earth.
Oh and Narutaki also becomes a cool dude and becomes Tsukasa's Tachibana in the end.[/hide]