Kamen Rider Kuuga (2000)

Both have main characters that make the world around them better. It feels like people's life's are improved by having met them. Both have iconography that invoke old civilization, while the ryusoulgers are part of the culture kuuga has to discover it's secrets by himself.

They're very similar in many ways to the point that Kamen Rider Agito was originally supposed to be a sequel of Kamen Rider Kuuga. They both have a dark tone to the series, though Kuuga is much darker.

Both are the start of eras for Kamen rider with Kamen rider of course being the first ever rider and start of the Showa era while kuuga is the start of the heisei era

(Kamen Rider Kuuga) Theme is Dark, it shows Kuuga evolving into a Rider and Ultimately didn't become what he was supposed to become and went on a journey to recover his smile. which is sad because he was such a happy go luck guy.

(Kamen Rider Ryuki)If you are looking for a Dark Drama with Mature Theme and you like to sob your little fangirl or fanboy hearts out, then this is for you my friends! I like it because it shows us all the dark side to us all with wanted to have a chance to fulfill a wish for a loved one or what have all the while you are fighting others and your inner self for the Final Wish at the end on that long and hard journey. The Finale will clearly satisfy those that truly love and stuck through the whole series! I liked it eventhough I shed some tears in the latter episodes.

Kuuga is technically the series after Black RX so if you want to watch the Kamen Rider series in order then watch Kuuga after Black RX.

Both shows are done by the same main writer Naruhisa Arakawa. The show's red ranger personality and philosophical views are similar to Kuuga's protagonist Yuusuke Godai. In both shows most problems get resolved creatively rather than overly relying on the toys that the shows are promoting. At a certain point in Abaranger, a main villain who plays psychotic games against the heroes is introduced which has superficial similitudes to the Gurongi in Kuuga.

Since Zero-One was the first Rider of the beginning of the Reiwa Era, you guys should check out Kamen Rider Kuuga cuz he was the first Rider in the Heisei Era. ( even tho both of these shows don't have the same comparison of the concept ) I thought of it because both Riders are the firsts of both Eras.