You're going to have to stay with me on this one...but let's talk about space for a second.
It's pretty clear that Oda has SOMETHING planned with space. Enel traveled to the moon in the cover pages (...and doesn't even need a helmet to breathe in space...hmmm) The Lunarians seem to COME from the Moon, or at least have a base there. There are Space Pirates. The Gorosei are literally NAMED after other planets. Something's definitely up with space.
And then there's the Planetarium we saw in Ohara. It had 5 planets surrounding the One Piece World. Which nicely parallels the 5 Elder Stars surrounding Imu. It's also worth noting that this apparently reflects real-world history. For a long time, we only knew about the 5 planets closest to us in the solar system, and originally we thought Earth was the center of the solar system. We didn't find out about the other 3 planets (Uranus, Neptune, and Pluto...which line up with the Ancient Weapons Uranos, Poseidon, and Pluton) until later.
So...is the One Piece World actually Earth or not? I find it especially interesting that Wano...actually has a MT Fuji. It's literally CALLED MT Fuji in the story. They don't really focus on it, but it's right there. So, I think it's VERY possible that the One Piece World is just a future-Earth that got Waterworld-ed.
BUT...I have a pet theory that I just find more interesting than that. Even though there is FAR less evidence for it.
Let's time out to talk about Inherited Will for a second. Inherited Will has become a VERY important part of One Piece's story. The idea that a person's desires can be so strong that they can survive after someone's death and be carried forward by another generation, either by inspiring them or possibly even influencing them more directly with Devil Fruits, is probably going to greatly inform the story going forward.
So...here's a question? Is it possible for a entire PLANET to "inherit the will" of a different planet?
What I'm proposing is...what if this ISN'T Earth....but instead a attempt to re-create Earth? What if THIS planet has "Inherited the Will" of Earth?
What if Earthlings...possibly having lost their home (more on that in a second) attempted to re-create their home planet on a different, possibly already inhabited planet?
They'd need the DNA of animals, and plants, and data on objects and elements and ideas that all made up the original Earth...which just so happens to be what Devil Fruits basically are! Or at least...the memories or ideas of all those things...
What if Devil Fruits are basically a advanced form of terra-forming a planet? A way of turning a non-Earth planet into something closer to Earth, which Earthlings could then live on?
Which potentially means...humans might not be native to the One Piece World at all.
So, let's back up again. I mentioned that humans might have lost their home-planet, Earth, somehow? Why would I think that? Well...let's talk about what I'm going to call...the TRUE Celestial Dragons.
The Celestial Dragons that we know in the story so far are a group of elitist, spoiled, out-of-touch man-children. They see themselves as "owning" the world and all the people therein as their playthings. BUT...they've also shown a interest in actual, literal dragons, like the ones we see being created on Punk Hazard.
And then there's Vegapunk's lab in Egghead. At one point, we see a Giant Robot opposite a Giant Antenna-sporting Dragon. A very ALIEN looking dragon, if I say so myself. And we later find out that the Giant Robot is a remnant of the Ancient Civilization. So that MIGHT mean that Dragon represents something, too.
So, what if our Celestial Dragons...actually took their name from the TRUE Celestial Dragons. A literal alien species that so happens to LOOK like Dragons....and who might be the secret villain behind all of One Piece. A Intergalactic Empire enslaving people across the universe!
Here's what I think happened: Earth found itself at war with these alien Dragons...and lost. It's possible Earth was destroyed. Humanity became the slaves of these Dragons. Somehow, a group of humans might have escaped and stolen some of the Dragon's technology...and figured out a way to create the Devil Fruits (They were probably CALLED something else, originally.)
This group of humans eventually find a planet suitable enough that they can transform into a copy of Earth...but there's already living, intelligent life on the planet. If I had to guess, I'm guessing the angelic Lunarians and the devilish Oni (Oar's race) were the two races already living on this world originally.
The Humans then use the Devil Fruits to fundamentally change the World. All the animals and the plants mutate into something a little closer to Earth, based on humanity's remaining memories and stories of the original Earth. Which...might explain why everything in this world is a *little* cartoony. It's all based on children's stories and faded memories.
Everything in this world is basically mutated...except the water itself...which MIGHT be why water affects Devil Fruit users the way it does. Maybe the water is the only thing that hasn't had it's "gravity" or "nature" mutated and is still a natural part of the original version of this world.
The Humans then establish the Ancient Civilization and some humans try to make allies on this new world. Some make strong friendships with the original inhabitants of this world. Some even join their pirate crews to explore this new world...like the Gorosei do.
But, of course, the Dragons eventually find them...but the Dragons offer the humans a deal. They'll let them keep their new planet...but only if they complete what they've done and get rid of the Lunarians and Oni, becoming the rulers of this planet. It's a sick joke by the Dragons. Either turn on those who took you in...or fight a war against a Intergalactic Empire they have NO HOPE of winning.
Some of their new friends, like Joyboy are willing to fight with the Humans...but most of the Humans lose hope and decide to take the Dragon's offer. Then begins the war. Most of the Lunarians and Oni die, but some are sealed away in a prison deep in the center of the planet. This is what's at the bottom of Enies Lobby, the hole in the ocean. It's a prison where the last remaining members of the original inhabitants of this planet still live...waiting to see the sun once more.
(Also, this is probably where I'm betting Big Mom and Kaido are RIGHT NOW. The center of the Earth is hollow...and it's full of "Devils"...or Oni, which both Big Mom and Kaido might be descended from)
So, the Humans form the World Government and rewrite history. There never was anyone else who lived on this planet. Humans were NEVER slaves. The World Government never did anything wrong...and THEY are the Celestial Dragons...there is no one out there they need to fear.
There were SOME Humans who sided with the Lunarians and Oni, though. They saved those they could. Some fell in love, even. These were deemed traitors to the new World Government and are marked with the D initial. We know the D means "Danger"...but it could potentially stand for "Demi" or half-breed. The joke being...after a few centuries just about EVERYONE on the One Piece World is a half-breed somehow. These were just the first families to betray the "purity" of humanity.
But, the truth is the World STILL isn't safe. Because if anyone ever overthrows the World Government, the Dragons will return and enslave them all again. The True Celestial Dragons have the benefit of time to wait for their toys to fail in their little game. So, whenever there is a light that threatens to alert the Dragons that the World Government has lost of control...it must be extinguished for the good of all.
TLDR? So...basically it's Gurren Lagann. With the World Government being Lord Genome and the True Celestial Dragons being the Antispiral. *shrug* What can I say, I like what I like.
I also think the Oni and MAYBE the Lunarians will end up being metaphors for the Ainu people. The original inhabitants of the island of Japan that the people from the mainland killed off and then turned into "demons" in their stories.
A list of punches delivered to the Ainu people by the Japanese ‘wajin’ government throughout history.
raestea.medium.com
But, that does raise a question...if all that's true...then who's Imu?