Motifs you would see made possible Kamen Rider?
 

My list still stands:

Dinosaurs
Ninjas
Martial Arts

I wonder if they could do the Zodiac, jewels, or light?

Zodiac was technically already used before. The villains in Fourze were all based on zodiac signs.
Light ocassionally appears as a minor part of powers, such as OOO flash of light in Latoratah Combo and Faiz' glowing stripes and eyes.

I believe I said it in Sentai, but I think it might make for an interesting KR theme too, Chess for the Rider motifs. Not the villains. This would give way for a female rider to be A primary focus, not THE primary focus, as a secondary rider because of the Queen piece.

Chinese zodiac, then? :p

Might not be impossible if we had chess piece villains in Kiva and already had cards
Might work for something like Ryuki, with a large number of riders.

Actually, when you think about it, the Zodiarts monsters were just based off of the constellations. The Horoscopes, I dunno if they were generals or whatnot, were based off of, well, the Horoscopes.

Kitty cats. Kitty kitty. Kitty kitty coo. Kitty kittyyititktkitktktittiktitktititlkdkgbjkykcfn

But yeah I want a cat.

Name animals all you want. OOO used them all xD

How about martial arts for the next kamen rider?

I'm thinking of something Blade style.

Four riders - four animal symbols vs the masters of the Chinese zodiac.