I quite liked Liveman's theme about the flaws of Japanese educational system and its overwrought social and psychological pressures, plus the classism of it. They should totally do that again.

Since Ninninger is new and that we'll wait for more info to see how KR can use a ninja concept from that, Kakuranger & Hurricanger....

Zyuranger, Abaranger & Kyoryuger's dinosaurs can work for the next rider.

Discs can work, but the kaijin are a problem. The secondary rider represents extinct beasts.