I'd like to see them try a musical theme again. Hibiki was unique but I think the music kinda got lost as time went on.

I'd like to see them revisit a Rider with only one element, ala Stronger.

It could work out like Firebrand's Fire Stones in Demon's Crest: One can used for offense and another for high speed and so on....

I'm better off with the monster theme of Kiva.

How about this?

Two riders, two monster slayers.

Main rider - a half-breed vampire (unlike Wataru). Was thinking on the lines of Blade & Donovan Baine.

Secondary rider - a monster hunter who got bitten by a lycan. Thinking Van Helsing & the Belmonts.

Female rider - a vampire lady (full-fledged)