My thoughs about one Piece world.
Regarding the daily disrespects made to standard biological/geographical/physical laws, I wouldn't be surprised if the world was a gigantic ship, travelling through space.
The safety system has gone wild from a long time, transforming classic earthian climate into what one Piece is (numberous islands with their own climat, monstruous creatures, etc.).
The world government is probably trying to hide it, fearing it could cause havoc and chaos when people will understand they're out of nowhere, with no way to control where they're heading.
The devil fruits would be recipents for nanomachines, rewriting the DNA with random capabilities. It would explain why you can't eat two devil fruits, because their respective nanomachines wil try to erase the modification from the others (leading to complete DNA destruction).
-> Bioshock Theory.
Another guess : the phantasy star-like ship is in control of powerful xenobeings, after mankind has lost the fight during the lost history, and they are using humans for experiments (the "be useful to us, or be extinct").
-> Dark City/X-Files/Matrix theory.
btw : Do we know what happened to Eneru and his travel to Fairy Vearth ?