There was once a kingdom called Ancient Kindom. It was a whole continent with mountains and cities that linked together.
Their inhabittants had advanced technology. But since they were so peaceful, they get lazy and clumsy. To make up for that, they make devil fruits to give them super strenght and powers. Since they lived in mainland, they didn't care much that devil fruit user was useless in ocean.
One day, a group of pirate attacked their kingdom. Thanks to devil fruit, they were able to defend their nations. The pirate, desperately weak against DF user, found a way to steal blueprint of Poseidon, an Ancient weapon from Ancient Kingdom so powerful that it can destroy the world.
The Ancient Kingdom didn't know about the pirate's plan. They go on living until the day Poseidon was activated. It created a big hole in the ocean which open a gateway for seawater to flow upward and flooded the whole continent and the Ancient Kingdom itself. Every year, the hole is opened and creats a new wave called Aqua La Guna that can sink any ship. The hole is called EL later on.
In a day the whole nation was flooded. 99% of its inhabitant died. Only those that live in the mountain survived. Weak and confused, people of once a big, united nation (ONE PIECE) was isolated, the mountains they lived in turned into islands. They were easily conquered by the pirates, who knows the sea well and good in sea battles.
The stones where the hole was opened called sea stones. They were under water for million years, but was destroyed by Poseidon, letting the water flow through, upward. The current was so great, it changes the surface of the earth completely. For one thing, it washed all the sea creatures (sea kings) all the way to grandline, where the current can't go anyfurther. It creates Aqua La Guna on the way there, and make the magnetism messed up in grandline.
Before falling to the hand of pirates, the Royal family of Ancient Kingdom flee away in a super boat called Pluton. They were lost but not lost hope yet. Since the EL hole was created by Poseidon, there is a way to reverse it, using the weapon again. But the weapon was destroyed by the pirates, and to make one again, they have to find informations from the poneglyphs. The blueprint of the Ancient Weapon was somewhere deep in the water, where Ancient Kingdom was. It's impossible to find the place in the middle of the sea, but the survivals from the Ancient Kingdom knows a way to pinpoint that place by connecting the islands, once was the mountains of Ancient Kingdom, to creat a map. To do so, they created the poneglyphs to point directions to the next islands (the next used-to-be-mountain) Eventually, the poneglyphs will point to the next islands and make a full circle around the world, along with grandline. To read and map them all, you need to travel along grandline. The only one who have traveled farthest into grandline was G. D. Roger
The pirates, after conquering the islands, make themselves governments, and united all the country and islands and call it World Government. The used to be pirates are now Marines.
The Royal survivals escaped from the pirate - now is the marines and WG. But they were not strong enough to take over the country again. They had to go into hide out and eventually changed their names. To make remembrance of the once Royal blood, they named their middle names D as in Descentdants or Defender. They forever carry with them the secret of Poseidon and WG, the Ancient Kindom that they call One Piece now.
Years later, a descendant of them, Roger, using Pluton, the ony powerful enough ship to travel grandline, and discovered the truth. Marines eventually gets him. WG could never have able to catch him, but ROger knew he couldn't taken the WG alone, so he turned himself in inpurpose, so that he will be executed in front of the world. Just before he dies, he tell the world about ONE PIECE, thus creat a new age: Pirate Age, in the hope that they, inspired by the story, will stand up and be strong enough to fight back the WG and find out the truth.
Knowing the power and the danger of pirates, since they once were pirates, WG tried to cover up the story in one-hand, and make a deal with the strongest pirates they can find - the Shikibukai, make them an oppresor of other pirates, and give them alot of privileges and power, turned them into hunting dogs. Full and satisfied, Shikibukai works for the WG, hold onto what they have, forget about One Piece, and kill any pirates who get in their ways.
EDIT: I combined the ideas. ENglish is not my first language so correct me if I'm wrong
But I have a gut feeling that this theory could come true.
dereshi dereshishishi ^^
