Submersible vehicles.

I don't think we've had a circus themed sentai yet.

Like a ringmaster, Lion tamer, clown, magician and two acrobats.

Muay Thai Themed Sentai

and

Wolf Themed Sentai

Gekiranger.

Lovecraftian creatures? Just kidding how about an anime themed sentai ( shounen red, shoujo pink, comedy green, sci fi blue, fantasy yellow, horror black)

Make them all edgelords :lolol

Sports with each ranger representing a sport.

Goggle V?

....I'd like to see a food-themed Sentai.......

I was gonna say that's stupid even for Super Sentai, but then I remembered we had a Kamen Rider fruit salad, a sushi morpher and a burger morhper. They could do it if they want.