[Sunred]

Thing is Takatera really pushed for a non-rider show because he had this idea of revitalizing non-rider titles by Ishinomori to create a MCU-esque Toei Hero World. and Henshin Ninja Arashi reboot was the first thing he had in mind. The reason he thought Arashi would be fine for a reboot was because he thought playing with altitude in terms of filming would enhance the quality of action scenes(the NTT Docomo commercial where the boyband SMAP was being Gatchaman seemed to have left an impression on him), and he wanted to ditch the armor look that riders have been going for and have a more smoother design. He got the approval in April to do a non-rider show.
But hearing that Justiriser will most likely be a ninja-themed show at the time they dropped the idea of a ninja hero and thought about going for a magical-themed rider at one point. Of course, then they got wind of Magiranger during pre-pro and dropped that too. Fun fact, they were planning to film the opening clip in New Zealand like Magiranger because they had this rough idea of heroes gathering from around the globe.
Around June, Bandai felt that with shows such as the Choseishin series and Ryukendo appearing, it would be dangerous to not continue the Kamen Rider franchise, and so the plan switched back to being a rider show, only this time they aimed it to be a non-conventional one. While looking for a new motif for this new show, they wanted to incorporate the idea from Arashi that he is a hero with ties to evil powers and settled on onis as the new motif. Around that time, Takatera was inspired by a taiko drum performance and added that to the mix which is why Hibiki's build is rather muscular. They later had ideas of sci-fi elements being incorporated and the onis originating from the same source as the monsters of the show, but Takatera finalized the villains being natural-occuring yokais, ergo the makamous and the riders as those who have the look of a oni but have no relations to the monsters of the show around August.

FYI, here is the SMAP Gatchaman commercial I mentioned for NTT Docomo, which is a Japanese telecommunications company