The Rain God and the Deep Deep Sea
The central claim of this theory is that the Yami Yami no Mi is not a logia of darkness at all.
It is a Mythical Zoan: Hito Hito no Mi, Model: Zaza—the Rain God Fruit.
What the world has long interpreted as the devil fruit of "darkness" is in truth the manifestation of the deepest sea, the irresistable and inescapable darkness of the bottom of the sea.

In our world, more commonly known as Davy Jones' Locker.

Why Not Dragon — And Why Nika Still Fits
The first reaction I expect is, that you assume that the fruit of Zaza should belong to Dragon, because we expect great things from him and he is already connected to weather phenomena, like heavy wind, rain and potentially thunder (logue town). Your

Okay, so I want to thank anyone who takes the time to read this theory. I’ve had this idea for some time now, with that being said I’ve never felt like I had the proper medium to share it in detail. Thank you to Morj for building the space.

Now on to the theory:

I think the One Piece is the group of people who lived in the Ancient Kingdom that Joyboy wanted to save, when he sent them forward in time with Lady Toki’s power he left behind stone tablets to ensure someone would be there to meet them. And I think the note Roger read in Laugh Tale was something along the lines of “Hope you made it there in One Piece!”

“We eventually realize that there once existed another country, nothing remains of it.” (Clover Ch. 395 Page 7) Over 800 years ago there