While creating new Riders and adding new forms may sound like a bad idea, think of it this way:
If the 10 worlds are indeed merging, events that already occured, such as who attains the Agito Seed, or if the Mirror World still exists, could be drastically altered as a result of the quantum mechanics being bitch slapped and turned upside down. This could be why there is a different Yuusuke wearing the Arcle Belt and Ichijou (his police officer comrade) is a woman now.
This could also create alternate outcomes, much like how the events of Kamen Rider Kabuto: God Speed Love can serve as an alternate to the television series. With everything being messed up, who knows, there could have been more ZECT belts created if events turned out differently. Maybe Den-O ends up with another form as a result of the storyline turning out how it was supposed to? Maybe another person with the Agito Seed became the main character because the merging affected the dimension and indirectly caused Tsugami to miss the ferry that day? Maybe Kamen Rider Blade ended differently for some reason or another, allowing Kenzaki to remain as Blade.
There are WAY too many possible changes that could occur when such BIG interdimensional events like this occur. Who knows how drastic the changes are... Maybe Den-O and Kiva are gir- er..wait, nevermind that one.
Its entirely possible that Decade assumes the other Rider's forms in order to stop the merging by fixing these sudden changes? I think i've said enough here...now to wait for someone to harass me over this lol.