One Piece Theory
So many years ago, Oda mentioned in an SBS that "a certain doctor" would one day tell us what the Devil Fruits truly are.
Vegapunk came close on Egg Head, but it has not been confirmed that he was that "certain doctor".
There have also been various mentions throughout the story that the previous user of the Op-Op Fruit was a famous doctor who had used it to perform miraculous operations and cured countless diseases, possibly even having given up their own life (likely involuntarily) to perform the Perpetual Youth Operation on Imu.
And so I put forth that this "certain doctor" is someone from before/around the Void Century, and that we will not only see them in the inevitable flashback, but also learn the secret of the Devil Fruits in the process.








