Lets think about the mysteries of One Piece
1)One Piece (the treasure)
2)The Void Century
3)The Forgotten Kingdom
4)The Will of D
Since we don't have any answers for these four lets put them together. Is it possible that "One Piece" is the name of the Forgotten Kingdom wiped out by the Celestial Dragons ancestors? And then all knowledge of it was wiped out, creating the Void Century of history. As for the Will of D, it's very likely that they are descendants of the kingdom's royal family; this could very well be why they are feared and hated.