In the latest chapter, we got the information of a treasure, which is supposed to be under Mariejois.
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A land above the Red Line :
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Now we know Raftel supposed to be a mysterious place, basically impossible to find. Raftel could hold a Poneglyph ( the last one ) That explains the history and where the treasure is and what is it for, Unless you hear the voice of all things, or read Poneglyphs, you can't read it ( Luffy/Robin ). ( Rayleigh explanation to Robin, even if she knew the truth, she wouldn't be able to do anything about it as she is now )
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What if One Piece is the treasure Doflamingo knows about. The Treasure
Doflamingo wants to find, the treasure that resides under Holy Land of Mariejois. Remember, Doflamingo doesn't want to become Pirate King. Therefore, ( Even if he knows its actually One Piece ) he has no reason to tell anyone about it. He wants to destroy the world on his own. He can supply all the Yonkou's with the " smileys " as he plans to use the treasure to destroy the world anyway. He knows no one will be able to find it, even if they find Raftel. He knows even if they get direction of the Treasure, they wont be able to get it, as it sleeps under the world Government's base which basically all the Pirates (even Yonkous), would stay away from. ( Stated in manga before )
Doflamingo could not care about how huge one Yonkou's army grows, As he has a different plan, While he gathers money and resources for his plans, he can sell of everything he wants, until the day he uses Ope Ope fruit, to grab the treasure, and potentially "Eternal Youth ".
Reason Corazon was killed, was probably because he didn't want to cooperate. DD decided to transfer the Fruit to someone else. Corazon had to die for not cooperating with DD and his plans. The moment Law cannot forgive - The first friend he ever had died for not cooperating with Doflamingo's evil ideas. Law took the fruit even though Doflamingo didn't want him to have it. (Stated in today's chapter). Law tried to fight DD as a kid, but he couldn't even get past Vergo's haki, ( As Doflamingo stated in the Punk Hazzard ) due to having no experience with the devil fruit
Whoever finds Raftel, and gets redirected to the treasure, Would have to destroy Red line, to get to the treasure, which is probably impossible for a human being to do. Law's Ope Ope fruit, could do the trick in this case. At the moment we're observing the time where Law's gratitude and respect towards Luffy will probably grow. In case Luffy is the one that saves Law, And revenge's against Doflamingo for him, as he himself would fail to do so. Law could arguably be prepared to follow Luffy and support him. As it seems at the moment, Law's dream is to revenge Corazon. He doesn't care if he dies doing it, As long as his revenge is done. ( Means he would even drop his dream of finding One Piece for it ) There's a possibility Law wouldn't care about being Pirate King afterwards, if it means standing in Luffy's way, the same way Old Whitebeard didn't want to be one. ( Dreams are similiar, in a case where Law has completed the goal he had and Whitebeard, having family - None are related to actually being Pirate King ). Back to Luffy who was fore-seen as Destroyer of the Fishman's island. We all know Red Line lay's directly above the Fishman's island. Luffy is forced to destroy Red Line, in order to get to the treasure, Red line potentially falls on Fishman's island, therefore Noah get its role, to transfer Fishman's on the surface without destroying the bound Luffy created with them.
Back to the treasure. The treasure ( arguably has the potential to destroy the world as stated in manga ) in this case being strong enough to destroy the entire Red Line could do the job, and as a result - Connects the entire sea's together. Forming One Piece.
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World government who represents 4 seas. Who's connection is broken, due to Mariejois standing in the middle, aka Government blocking them a way to connect. ( Red Line )
Mariejois falls, Government falls. A moment for Dragon finally arrived. The Moment Dragon was waiting whole life for - To get the World Government on the ground. Time where the whole world goes in a war. The connection between the seas is broken, All the allies Luffy met so far, has a free way to assist him - Even the first half of the Grand Line ( Aka Alabasta - Drum, Sabaody, Fishman's etc ) . Everyone meets up in the middle
Revolutionary Army vs The World Government
Pirates vs Marines