[Suzu]

The thing is that Ryuki could have been anything, it doesn't have to be Kamen Rider. Didn't they plan it as a Metal Hero show at first?

Kuuga was a show that came to be as the comeback for Kamen Rider and Ishinomori took part in the production during his last years (concepts and such).

So if Kuuga was explicitly planned to be Kamen Rider and Ryuki was just some ideas, Ryuki is more a trend of the time for toku, something that would eventually come to the franchise anyway. For example, the Rider vs. Rider already existed in Agito and the Hero vs. Hero was also in Sentai before. Agito also had multiple Riders, just not as many. Toy sale focus was in Sentai since Gaoranger etc. Ryuki was more of a gate for those elements to be established for Rider, rather than an innovation.

At least that's how I view it. Also, unlike Kuuga and Agito, which opened the path for Heisei Rider as we know it, Ryuki didn't set a certain standard. Faiz and Blade have more in common with Agito than with Ryuki, from a mere concept standpoint.