Joy Boy vs the World Government : Battle of the “D” tribe vs the World Government 
From the long name of this theory there is a lot to explain so let’s start with the timeline of this theory: Joyboy existed around 800 years ago at the time of the Void Century.

I’m going to write down the contents of this theory:
1) Void Century
2) Tribe of “D”
3) Adventures of Joyboy
4) Fishmen Karate vs Devilfruits
5) War between the tribe of “D” and the Fishmen vs the World Government

Let’s begin

Void Century


The Void Century is a century-long gap in recorded and archaeological history, the study of which is forbidden by the World Government. 
These events occurred 900 to 800 years before the current storyline.

The Void Century because there is nothing left to acknowledge anything that happened in it, aside from the Poneglyphs which are written in a language that few know how to read.

However, it causes a problem with the known histories of the world as nothing ties up because of that missing period. Though many seem to know there is a gap in the historical record, virtually everyone is unaware that filling in this gap is possible. For example, Nefertari Cobra, after being told by Nico Robin about this missing period, was completely shocked that a way to “tie the threads of history together” existed. The fact that filling the gap is possible was proven when “Dark King” Silvers Rayleigh revealed that the Roger Pirates discovered the true history.

Tribe of “D”

This is the most important event during the Void Century.
As who was in this war is unknown the war was between the Tribe of “D” and World Government. The “D”s could have been the tribe that was in the New World but the World Government only came to power after the Void Century.
So the World Government had the Devil Fruit Tree and they used these to wipe out the tribe of “D” to rule over the world as the “D” could have possessed the Ancient Weapons that the Government needed to rule the World and so the war began but the devil fruits had its ultimate weakeness, sea water “become a hammer underwater”.
The Tribe of “D” could be the ancestors of the current “D” meaning they that they would be Monkey D. , Gol D. , Marshall D. , Trafalgar D. , Portgas D. and Jaguar D. So these were the 6 families that has been left but Jaguar D.Saul could be the last of the Jaguar D. Family and also since Gol D.Roger is dead and Portgas D. Rouge and Portgas D.Ace is also dead those 2 families could also no longer exist but if Ace had an unknown baby before he died, both Gol D. and Portgas D. could still exist


but the Government’s plan is to extinguish these families who oppose the Government but the only families that are left would be 3 families that would be Trafalgar D. , Monkey D. , and Marshall D. And 5 people with “D” still exist and they there all extremely strong since:
Monkey D. Dragon is the most wanted man
Monkey D. Garp is a Marine Hero ( but he is Marine so he isn’t a target )
Monkey D. Luffy is a pirate worth 500m
Marshall D. Teach is a Yonko with 2 devilfruits
Trafalgar D. Water Law is a pirate with the Ope Ope no mi ( worth 5Billion ) and also a bounty of 500m

So these guys are what’s left of the “D” Tribe so the conclusion is the world Govenment Tried to kill off the Tribe of “D” who had a Ancient weapon with Devil fruits reason for the war

Adventures of Joyboy 
I believe that Joyboy was also a member of the “D” Tribe and Joyboy could have been a cover to hide his acutual name like Marshall D.Teach his name under the alias Blackbeard.
So Joyboy is an alias that was used to cover up his real name which had a “D” in it.
I mentioned that there was a huge war, I think the the World Government stole the weapon that was in the possession of the “D” tribe so Joyboy went on an adventure to look for a the other ancient weapon known as Poseidon , a mermaid that lived in the Fishmen Island. We’ve seen princess Shirahosi use her powers to communicate with sea kings and this is deadly enough but the main reason for his adventure was to bring fishmen to the surface world to fight the war against the World Government ,
But why fishmen ???

Fishmen are masters of Fishmen Karate. Fishman Karate is a martial art that is practiced by certain skilled members of the fishmen race.
According to Jinbe, the secret of Fishman Karate is mastery of the water in the user’s direct vicinity, using its power to send powerful waves impacting the water within the opponent’s body. Because every creature has a high percentage of water in their body, the users of Fishman Karate are able to battle opponents that are usually highly resistant against blows and punches. This was demonstrated against Monkey D. Luffy, a rubber man who is nearly immune to blunt attacks.
The Government sent devil fruit powered people to kill the “D” so Joyboy wanted them to help the fight.

Joyboy was a member of the “D” tribe he was involved in the war during the Great War but they were out numbered by Devilfruit users so Joyboy went to Fishmen Island to ask the fishmen to help them fight the devilfruit users.

Fishmen Karate vs Devil Fruits
Now I just explained the origins of Fishmen karate but this is a key factor in this theory since Fishmen karate is able to use waterbased attacks.
If a battle is on sea against devilfruit users and a Fishmen can use Fishmen karate, the Fishmen can win .
Here’s the reasons why:
• Rain cannot effect Devil Fruit users but the water based attack’s we’ve seen from Jinbe can weaken the devilfruit users;
• Fishmen are 10x stronger than humans;
• The Sun Pirates were able to destroy a marine fleet ship easily because they can do a surprise attack if the are under the boat.
So Joyboy wanted fishmen to help him in the war.

The Great War : Tribe of “D” vs The World Government 
We know that Joyboy was unsuccessful in bringing the Fishmen to the war so the World Government won as the 20 kings moved to Mariejois. They could also have gained the mysterious anicient weapon Pluto.
But the World Government was unsuccessful in erasing the Tribe of D. as the families I mentioned earlier could have escaped and Joyboy later returned back to Fishmen Island because he is unsuccessful in bringing the Fishmen with the Noah so he delivered the Poneglyphs.


Joyboy could have been the first Execution that the World Government did to show their power. 

Luffy and Roger might have had the will of Joyboy. 
*Theory by King of beasts kaido

One Piece Family