A lot of the Kamen Riders appeared in Kamen Rider Decade, but I also noticed that quite a few did not appear in the show nor the movies. I am counting Kamen Rider Decade the series, Cho Kamen Rider Den-O & Decade NEO Generations: The Onigashima Battleship, and All Riders vs. Dai-Shocker. Now, there can also be a chance in the movie coming in December.
Another Agito did not appear in Kamen Rider Decade. Many of the Oni Riders from Kamen Rider Hibiki did not appear in any form, including Sabiki, Eiki, Danki, Gouki, Toki, Shoki and Banki.
Certainly Shuki didn't appear in Decade. It's weird because Kamen Rider Shuki's stunt actor Naoki Nagase was working for Decade but didn't reprise his role as her. He reprised all his other roles (Psyga, Glaive, Tiger, Ketaro, Punch Hopper and Sieg.) Transformed Kyosuke also did not appear in Kamen Rider Decade. Kyosuke briefly transformed into this form. In stageshows, he is called Kamen Rider Kyoki.
Kamen RIder Kabuki of the Hibiki movie was summoned by Diend but the other movie-exclusive Riders of Hibiki did not appear in any of the Decade properties. Touki, Kirameki, Habataki and Nishiki above.
Correct me if I am wrong, I am not sure about these. Neither am I sure about Gaoh or Arc. I am sure that Nega Den-O did not appear in any. Kamen Rider Yu-Ki did not appear either. Additionally, Riderman appeared in All Riders vs. Dai-Shocker but Tackle did not, but she was never an official Kamen Rider.
Related Topic: Kamen Riders to Physically Appear in Kamen Rider Decade