one of the things I find is missing from this mod is Dagon
there is no Dagon statue and no related items
it would be really cool if there was a Dagon temple similar to the ocean monuments that is found deep underwater in oceans and can contain useful items for the overworld

Since the current roster of Great Old Ones is quite narrow, I intend on adding more of them. Probably a good time to start working on adding more now, so that's gonna be a thing.
Since I plan on overhauling structure generation in the Overworld, adding some form of temple sounds quite fitting.

I'd love this. Minecraft's oceans are frustratingly empty and pointless - finding little aesthetic/lootable ruins on the ocean floor would be amazing by itself, let alone the joy of a full-fledged temple with stuff in it. Deep Ones, maybe? A Terrible One giant squid-Guardian hybrid?

This would be great, and maybe add in the ruins of R'lyeh with Cthulhu's tomb, since that's supposed to be in our world.