Akira (ToQ rokugou) is a shadow. If he's defeated will he become giant like normal shadow?

In theory, yes? :lolol

I'd like to see a sentai in the lines of Granzort/Rayearth.

Three girls get magically transported to a fantasy world where they effectively have to kill off an entire royal court until it turns out that this makes them the villains in on story somehow and their whole mission to protect the princess was also to kill the princess?

I wish Gekiranger was better received in Japan...

What if Sentai have more approach to PR in terms of plot ?

Is there any season that is as dark and serious as Go-Busters?

Liveman?

I'm not bug with the theme from the sentai 2019 besides, it will be interesting to see a dragon-themed sentai.

Like what Gosei said, Liveman is a safe bet. A few others that are not 'dark' but have somewhat of a serious tone overall that I can think of at the top of my head would be Flashman, Maskman, Jetman, and Shinkenger. But Liveman for sure.