According to a popular theory, One Piece is a way to gather and use the Ancient Weapons to destroy the Red Line at Reverse Mountain and Mary Geoise, connecting all Four Oceans into One Piece – fulfilling Madam Shyarly’s prophecy as well as many of the Straw Hat Pirates’ dreams (such as Sanji’s desire to find all Blue).
I believe this to be the truth due to the recent references to Poseidon, including Roger’s statement that it was too early for him to accomplish it because Shirahoshi had not yet been born.
I believe this is supported by the term “making the whales happy” in the paragraph titled “making the whales happy.”
After the great battle, the establishment of the Red Line destroyed the whale population. Baby whales captured in Reverse Mountain’s stream are separated from their pods and sent to the Grand Line with no means to return (like Laboon).
Furthermore, migration routes were barred. Whales can also be spotted around Fish-Man Island, since they are attempting to migrate but become stranded there. The demolition of Reverse Mountain will allow these children to rejoin and normalize the migration pathways of the whales. I believe that the Pirate King and The Sea Kings will collaborate to destroy it.