I have an idea, but I'm going to refine it. It's been done before, but it was recycled into In Space.

Universal Monsters!

How cool would it be for a Ranger team to have the powers of The Wolfman, Dracula, Frankenstein's Monster, The Creature From the Black Lagoon and The Mummy? Just imagine the mecha!

I would like to see another ninja team or ghost hunters. I think both could be entertaining

Oh man Ghostbusters Sentai!

Man, that just made me think of the Ecto-1 transforming into a giant robot :lol

So good man, so good.

Ushikai Sentai Nokaman: a Farming theme!
Oh wait, that's my fanfic... I got to get back to that someday.

And on the castles and Knights theme brought up, I think that'd be awesome and I do agree you can have knights without heavy Rider-style armor and history has shown many a knight (or other similar warrior classes) has worn light armor.

I'd love to see a team based around certain mythologies.Norse,Greek,etc. Can you picture a Norse Mythology.Their home base is Valhalla. The sixth ranger is based off Thor,the leader/commander is based off Odin.

Norse also has a lot of cool animals for mecha, like Sleipnir, the 8 legged horse, or Hraesvelgr, the corpse swallower.

Plus it'd be nice to have a religious bent sentai, that sticks to its guns. The Goseiger's are only angels in name, and the only reason I get annoyed by the cheap religion trimming in Japanese media is most religious texts are fantastic stories. Too good to be used only in imagery.

Whether you believe in them or not, they are pretty much the first comic books, full of monsters, and super powers and crazy mythologies if you went ahead and just did a straight adaptation of any of the religious texts and put some pretty Japanese men and women in spandex I'm about 1000 percent certain it'll work beautifully.