I would embrace the Japanese-ness and state the villains are ancient demons who were sealed away by powerful samurai in ancient japan (use the flashback footage we'll apparantly get) and that whatever was used to trap them was carried to where the Rangers are now (I myself would LOVE the new planet idea) and get somehow released.
The new kids get drafted to carry the power of the ancient samurais due to some sort of random reason (genetics, matching birthdate, current owners of the Origami beasts, what have you).