[Sunred]

While on the subject of Ryuki, the three riders in that show, specifically Ryuki, Knight, and Zolda each have one iconic element of a generic Kamen Rider helmet.
Ryuki has the eyes, Knight has the crusher(jaw/mouth piece), Zolda the antennas/horns.

Stronger's bike Kabutolaw is a portmanteau of Kabutomushi and Outlaw.

Agito and G3 together can be a nod to V3 and Riderman in a way considering Agito and V3 are both the second show of their respective era and G3 and Riderman do not transform in the typical way. The two also rely heavily on their tools.

Blade and Gaim both are indigo and have swords as the default weapon.

Leangle and Ryugen are both green and have dabbled in the ways of the evildoer at some point in their career as a rider.
They are both teenagers.

Zeronos and Airi(Ryotaro's sister)'s story in Den-O is that of the Hikoboshi and Orihime lore, a popular one in Asia that tells the story of how the Altair, Deneb, and Vega stars came to be. Zeronos' position as Hikoboshi is further solidified in his bull motif.

Kiva takes a lot of cues from Kuuga.
Kiva and Kuuga Mighty are both red and defaults.
Kiva Garuru and Kuuga dragon are blue and swift forms.
Kiva Bassha and Kuuga Pegasus are green and the marksman forms.
Kiva Dogga and Kuuga Titan are both purple and the strongman forms.

OOO Tatoba combo's color scheme seems like a reference to Pan-African colors, in this case red, green, yellow.
OOO Putotyra seems like a reference to Evangelion 1 in its choice of color and the fact that it goes berserk.

Gaim's Movie Taisen opponent is voiced by Shinichiro Miki the voice of Sieg in Den-O and the Genesis Driver in Gaim.