[Animeking1108]

I was going to suggest a Greek God theme too. However, my suggestion was having the Black Ranger represent Hades, the Sixth Ranger represent Hercules, and the mentor represent Zeus. The villain would be Tantalus, and the Bigger Bad would be Chronos.

Another idea is a detective theme called "Metenai Sentai Nazoranger." The villains would be a Yakuza/Mafia crime organization, and the Sixth Ranger would be a neutral Cat Burglar that would later join the team permanently.

My third idea is a chess themed season.
Red- The King.
Blue- The Bishop.
Black/Green- The Rook.
Yellow- The Pawn.
Pink- The Queen.
The Sixth Ranger- The Knight.

A computer themed season that is basically an homage to Tron. The Rangers fight monsters/viruses by entering a computer world. The viruses are being lead by a corrupt AI (much like Venjix from "RPM"). The Sixth Ranger would be a Trojan Horse virus that eventually let's the kindness of his comrades turn over permanently.

Finally, a military/war theme. The Mechs/Zords would be based on military vehicles.