Reuse a old theme like ancient civilizations ?

Of course that's more like it.

That would be pretty lit, although I feel that that theme isn't something that people nowadays (especially the Japanese audience) like... I think another rescue theme as mentioned before would be good i guess

But would be the villains this time ?

Hmm... I'm not too sure, maybe alien monsters that wants to take over the world like usual?

I prefer European mythological beast theme because I like it so much.

A aquatic-themed season will be great.

I think I’ve seen a fanmade webseries about what you just said. If I am not wrong it is called Scubarangers and it is on youtube.

How about yokai theme sentai that's more like it.

The name would be yokai sentai taikuger that's a exelent title I love it.