[Aoi Kurenai]

I think the franchise would be pretty close the where it is now but it would have taken longer to get there. Kamen Rider Ryuki gave us a lot of things that were pretty monumental - a Kamen Rider being the final enemy, the multiple Riders, Riders who have their own unique, often selfish, reasons for fighting. It added an interesting layer of depth to the franchise, building upon the dramatic character interactions Agito gave viewers.

But yeah, Ryuki was, at least very early on, a Metal Hero series, it has a couple of traits of the franchise as well: giant ally monsters and heroes fighting in different dimensions.