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.

As a running joke, I have Super Sentai Poweranger for the 40th series. All the rangers would have popular Japanese names for the time and the 6th ranger will be an American with a name like Gregory Johnson or something.

As an actual theme, Metal Sentai Rock-uger where all the rangers are different styles of Rock/Metal Music and they fight against a villain known only as The Man. It could have a villain naming theme a'la Goseiger/Jojo's Bizarre Adventure where they're named after some big name music acts.

Red: Heavy Metal
Blue: Speed Metal
Yellow: Power Metal
Black: Alt. Metal
Pink: Death Metal

6th Ranger: Nu Metal

I don't know much about Metal music, this was just a little idea a few friends of mine had a while back.

Boukenger was mostly construction vehicles.

Power Rangers Game Knights, just in Japan with a different name.

How about a steampunk theme?

we have steampunk villains this season

But I want steampunk heroes.

Boukengers mecha did consist of some construction vehicles but that wasn't what it's theme and design primarily focused on.

A voltron/thundercats.

Red-lion
Blue-tiger
Yellow-panther
Green (female)-cougar
Pink-lynx

Jetman remake

I have a couple ideas that could be feasible in the future.

The first one would be based on the planets and the team would be called SolarRanger.

Core rangers:
SolarSilver - Mercury
SolarPink - Venus
Solar Red - Mars
SolarYellow - Saturn
SolarGreen - Uranus

6th Rangers:
SolarBlue - Neptune
SolarOuja (pun on the word Soldier) - Jupiter (Black with Gold armour, which can be used by other the other rangers)

Extra Hero:
Ceres Knight

The primary villains would call themselves 'The Discord Trinity' and are Sedna, Eris and Pluto, and around the finale, after being revived, would merge into 'The Nemesis' (named after a hypothetical star in science).


My second idea would be based on the twelve signs of Zodiac and in the end there would be twelve rangers. I haven't figured which sign is which colour, but it's a possibility.