I didn't even realize BBTS still did sentai preorders. I've been ordering over at Tokullectibles but wouldn't mind having them in my pile of loot instead.

Looking at the ant, which appears to be a solid black piece, I'm thinking the designers on this one decided to make what would otherwise be a separate parts-forming piece (I'm thinking along the lines of the stuff that snaps off the back of the Zenkaiger jet to stabilize the legs in combined form) into something with a little more personality. Most likely, it'll be used as an intermediary part necessary to later combinations.

I can't help but wonder if God Kabuto might not form a mecha in addition to a big, awkward gun. That prominent gold color - vac metal, even! - is very Sixth Hero.

I thought I read somewhere that each Kingdom has a Diety they worship that becomes a weapon

I may be in the small minority here, but I really really loved the size of DX Kirameijin, and was hoping that all the DX robots would stay that size going forward... I bought Don Onitaijin, and I like its design and gimmicks, but the size I hate--way wayyyyyy too big. Plus getting Toradoragonjin means the investment in storage/display space is huge too. I'm glad this guy seems to be a smidgen smaller than those two figures, but I am still disappointed they're staying in this roughly "30 cm tall" range.

So if 6th ranger isn't a rhinocerous beetle what is it

My guess is King Crab.

A crab seems like a bit of a stretch for an insect theme. Yeah, they're arthropods, but at least I can understand why spiders and scorpions are normally lumped in with insects.
Still, it'd be interesting if it were a crustacean - at least unexpected, I guess! I'm personally betting on either a scorpion, moth or some kind of beetle. Maybe a scarab.

Also I hope the God Kabuto confirms that we won't get a Gold sixth again. Toei/Bandai really need to chill with that color.

Yeah arthropods like crabs are nicknamed "sea bugs" but I do like the Scorpion idea tho.

It was rumored for quite a while the 6th would be a darker purple spider, but I don't know if that's still in the cards.

Hercules Beetle is still an option.
Giant Isopods are strangely popular in Japan. And while crustaceans and not insects, they share numerous similarities.
Grasshopper is a potential candidate, too.
Firefly/Lightning bug would also be a neat option to roll with.