That's a possible theory if you look at Wano. At the end of the arc, Robin is shown what is an older version of Wano from years ago until the water rose too high. Its possible that years prior in the time of the unknown that the world was much like ours in several large masses of land surrounded by water and that all of the islands are technically part of those land masses.
Maybe it has to do with the weapons and One Piece being a terra forming weapon that manipulated the planet to make what it was today. Although it wouldn't explain why Roger and the others were laughing. Its also possible that the One Piece isn't that weapon and that the other weapons are and that One Piece is something all completely different. We do know that one weapon can manipulate the Neptunians and potentially destroy the world.
I also think it's related because of the coinciding theory that Imu (or Umi) represents the sea. Could it be that Imu is legitimately the God of the Sea, and is the one responsible for the cursed nature of devil fruits? Some entity has to resent the dreams of mankind quite strongly if the sea so detests devil fruit users.
It also makes sense that the celestial dragons would live high above the world along the Red Line if the ultimate goal is to flood the world again, making the Red Line the new island chain.
IDK, the red line has always fascinated me. Ever since the first time it was shown way back in the day I've wondered why the world is the way it is, and why nobody seems to take up residence along the Red Line. Then finding out that the world nobles live there made all the sense in the world.
---
(He/Him) I write Naruto Fanfiction. But I am definitely not a furry.