Wishful Thinking: Slicing Like A Ninja Themed Kamen Rider!
As much as I don't like Ninninger for how it turned out but I thought the ideas revolving around the Final Shuriken and the Last Ninja aren't bad ideas. Just that, Super Sentai may have been trying to stay afloat after ratings dropped from Go-Busters up to ToQGer according to @Wiki. I thought about this one... Naomi Takebe was responsible for some of my favorite Kamen Riders namely Kamen Rider Kiva, Kamen Rider OOO and Kamen Rider Gaim. For Go-Busters, I would think she's got her lackluster performance there and even if I'm among a few who like the series, you can't deny that she's not cut out for Super Sentai.
Yamaji Touha/Jiraiya appeared, as a guest star in Ninninger as the main event. Metal Hero had one ninja themed show, Super Sentai had already three ninja themed shows but Kamen Rider had no official ninja themed Kamen Rider. I think it would be good to try and revive the ideas of the Final Shuriken and Last Ninja in another Kamen Rider battle royale or just make a ninja-themed Kamen Rider. But I would prefer a ninja battle royale. This time, the fight for the Final Shuriken will grant power to the winner and a series of ninja-themed Kamen Riders (both good and bad) must fight for it.
The idea of a ninja battle royale may still need some polishing though. Kamen Rider Ryuki was very last 2002 and Kamen Rider Gaim was last 2014. That's a total of twelve year gap. We already have Ryuki and Gaim to tell Toei what to do and what not to do. Hopefully, we'll see a ninja-themed Kamen Rider.