Will the Next Great War Cause the World to Sink, as Predicted by Dr. Vegapunk?
During the Egghead arc, Dr. Vegapunk (Stella), while broadcasting, opened up about the great war that took place 800 years ago, concluding the Void Century. As a consequence of that war, the world sank by about 200 meters. Then, he talks about the Mother Flame that was used in Lulusia, which caused the sea to rise again by 1 meter. He argued that the world is still sinking with the rise in sea level. However, if we look at the Fishman Island Arc, we see the Poneglyph of apology. That apologetic Poneglyph was referring to Joyboy's failure to bring the Fishmen to the mainland, for which he prepared the great ship Noah. Secondly, I will argue that the Ancient Weapons were used by the 20 Monarchs that later formed the WG, which maybe resulted in Joyboy's failure.
So, here is my thought: maybe in the next war, if it happens, there will be no use of the Ancient Weapons because that will again result in the sinking of the remaining islands or continents, leaving no surface area to inhabit. Therefore, these weapons will remain unused and underwater, in the case of Wano. Whereas, in the case of Shirahoshi, the Mermaid Princess, she may use her powers for resurfacing the main islands and sending the Sea Kings to the ocean depths. Also, if BB gets the information about the Ancient Weapons from Caribou, isn't it possible that he will use them for negotiating with the WG for a much greater value? And if BB gets the information, then of course he will try to get his hands on them to become the true Pirate King and World King without the One Piece (in case these weapons are not part of the One Piece treasure itself). So, here is what I think: in such a case, BB will be defeated by the Straw Hats.
Moreover, Luffy as Joyboy aims to liberate the masses from oppression. So, if the story continues in joyboy's aspect and during the war the world sank again wouldn't that be another failure of Joyboy?
With that said, I also think that Shyarly's (the merfolk's) prediction will come true: that Fishman Island will be destroyed because of the struggle to take Poseidon (Mermaid Princess), the Ancient Weapon, and then Luffy, as Joyboy, to protect them, he will take the Fishmen above the sea to the mainland, fulfilling his promise. After the mainland is protected or liberated from the discrimination and slavery of the Fishmen and others.
So, I think that in the next great war, the world will not sink.
PS: I am having all these thoughts and sharing them to further my perspective and assess different dimensions of the story that I might not have caught.