Joy Boy was a man from the surface world who played an important role in the history of Fish-Man Island. At some point, he came across an island, which would later be named Laugh Tale by Gol D. Roger, and left behind a treasure.
It has been implied that Joy Boy was an extremely relevant figure of the Void Century, with Gol D. Roger declaring that he wished the two could have lived during the same period upon learning the true history on Laugh Tale.
So it stands to reason that the Great Kingdom, Joy Boy, the Ancient Weapons and the Will of D. are all connected, in fact that’s pretty much confirmed in Chapter 967.
It is assumed by many fans that Joy Boy was a very important person in the Great Kingdom, perhaps even its king.
The Clan of D. was a very important clan in that Ancient Kingdom (possibly its rulers) and the Ancient Weapons were created by, or at the very least were in posession, of said kingdom.
It’s very likely that Joy Boy is the person who did wear the massive straw hat kept inside the vault in Mary Geoise.
A very popular theory is that the Ancient Kingdom had plans on destroying the Red Line, thus promoting the unity of the world in “One Piece”. That of course ties back to the fall of Mary Geoise and the prophecy of the destruction of the Fishman Island aswell as the “promised day”, on which the merfolk will live on the surface, under the sun together with the humans and all other races.
Chapter 967 has confirmed what most fans have already pretty much known: that the One Piece was already there when Roger got to Laugh Tale. Therefore it was left there by Joy Boy and the Ancient Kingdom (It’s still possible that Roger had included something on the treasure, though).
For the One Piece to “function” or to work properly, it is believed that the three Ancient Weapons are needed. That’s most definitely one of the reasons why Roger couldn’t fulfill the will of that Ancient Kingdom, for he didn’t have enough time. After all, Shirahoshi would not be even born in around 10 years.
The One Piece is stated to be something tangible, meaning that it’s not something figurative, or anything like that. Whoever finds it learns about the True History and becomes the King of The Pirates. It is obvious at this point that the One Piece is at Laugh Tale.
There is a theory that the Ancient Kingdom was a kingdom of pirates, only that these pirates were simply adventurers that sought to explore the world and spread that kingdom’s ideals of freedom and equality throughout and between the different races of the world, making several allies in the process: the merfolk, the minks, the Kouzuki clan, Shandora, etc, simillar to how Luffy has been gathering allies throughout the story and has been freeing entire islands from cruel leaders and freeing people in many different ways: from literally freeing people from a prision or from their dark past, to freeing kingdoms from a fabricated past (Dressrosa), from a tyranical leader (Skypiea), or from imminent collapse (Alabasta)… it’s not a surprise that the theme of freedom is one of the most prominent in the One Piece story.
It is assumed that the Ancient Kingdom is Laugh Tale itself. This is further evidenced by the fact one learns about the True History and everything when they get there, aswell as that it was Roger himself who named that island Laugh Tale, which means it was not its original name.
When professor Clover spoke about the Ancient Kingdom, he said:
Since he was shot just before he was about to speak the name of the Kingdom, it stands to reason that the name was extremelly significant, not only to the characters in the story, but also for the fans reading it. For many years people have wondered and speculated what that name was, the answer is very simple: The One Piece is the Ancient Kingdom.
The one who finds the One Piece becomes the King of the Pirates, if the Ancient Kingdom was a kingdom of pirates, that makes completely sense. That also explains why Clover was cutout before saying the name of the Kingdom, because that would reveal everything:
“The most essential thing that holds the key to everything is the name of the once prosperous kingdom.”
The one who finds the Ancient Kingdom becomes its king, and, in possession of the Ancient Weapons and after gathering a multitude of allies, they can destroy the oppressive World Government and establish a new Dawn, where that kingdom will rise again and all races will be part of it. An united and equal world. “One Piece”.
Of course, that also ties back to the theory of the destruction of the Red Line for a united world, literally and figuratively.
“Wealth, fame, power.”
The title of King of the Pirates was, up until this point, always treated as something figurative. “The person with the most freedom”. While in reality is quite a literal title. The King of many nations, with their people and ideologies. The one who carries the will of that Great Ancient Kingdom and that will unite and lead the entire world into it’s Dawn. We know that Luffy isn’t interested in conquering anything, so we will have to wait and see how everything will unfold whenever he becomes the King. Which path is he going to take the world, if he is interested in ruling at all.
*Theory by Andyrion