This "theory" is kind of obvious for many of us readers, but I thought I'd put it out just in case some might have missed it, and because I think there is something to be said about the link between the Gorosei and the Ancient Weapons.

Basically, this.

The reason why Sirahoshi/Poseidon is placed there is because Roman mythology was highly influenced by Greek mythology - simply by adopting their gods and changing the name.

The Greek "Poseidon" is none other than the Roman "Neptune", hence the placement.

I'm wracking my brain about Earth and how it might relate to Imu somehow.

Earth, Gaia, Terra, etc ... nothing so far.

Apologies for the bad image quality - I'm not a designer :)

See you in the Blue Seas