Some background info -
Goranger, the first Super Sentai show, you know, Power Rangers, was first pitched as a spin-off of Kamen Rider. Featuring a team dynamic of five Riders. But producers didn't think it would sell, so the creator retooled it as a separate entity.
For the (now regular) crossover movies that Sentai does with the current Rider show - they've dug the concept out of storage - and given it a three part miniseries. They still don't think it can stand on it's own, so they're having a bunch of past Kamen Riders guest star...but still - neat.
Kamen Sentai Gorider