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I think the recent era of One Piece is post apocalypse, the WG and Celestial Dragons took advantage of the situation using the remaining knowledge what the remaining void era have right now to control the world. They have been searching the hidden weapon that have the possibility to save the world from sinking, is Oda implying that the Pluton is Noah's Ark as the other ancient weapons could control the ocean and the sky. Joyboy and the rest of remaining species are fighting against the 20 nations to stop their corruption while they created the Ancient Weapons to protect the world. Why would the Celestial dragons and the government allowed the slavery and racial discrimination, destroying every nation who are against them, it's because they are power hunger people who want to take the ancient weapons for themselves, to be Gods.
People with "D" in their names are must be the descendants of the first generation pirates. After Roger find out the truth, he left the message for the new generation of people carrying the "Will of D" before his death so they can look for One Piece to stop the WG and their radical reigns.
That's my theory.