[Matrixbeast]

An anime set during the Showa era would be cool. Spirits adaptation or completely original would be quite nice to see.
Would give a chance for some of those bulkier suits to get more dynamic action.
There's a lot of stuff they could possibly do, like write a fleshed out story for the Shocker Riders, or have Amazon get more time in the limelight.

For stuff on the list, Hibiki is a strong contender. There were so many Oni that had one or two appearances that a series that follows other members of the Takeshi organization would be cool.
Either that or have the original ending and let Asumu become an Oni.

Another choice would be Ghost. A deceased main rider, a world of spirits, and no more budget restrictions, Ghost had the most potential that the yearly KR budget just can't do it justice.
However, considering Ghost's biggest downfall wasn't it visual department, but it's godawful writing, they would have to change things up to a crazy degree for it to be watchable.

Maybe use the early Agito draft of it being a continuation of Kuuga, and have an alternative depiction of the modern day double Riders, as well as have animated depictions of the G1 and G2 systems.

Speaking of alternate depictions of the double Riders, ZO and J were designed to look so similar to emulate how Ichigou and Nigou looked so similar. Maybe figure out a way to tie their stories together?