I have to say, the idea of One Piece being not treasure, but actual written history about the Void Century is an appealing one.
For one, it'd make sense why Roger was such a threat to the World Government. And it'd make sense why he encouraged so many people to go after it. After all…what can be more powerful than KNOWLEDGE.
It's KNOWLEDGE that lead to the WG to destroy Ohara. It's KNOWLEDGE that lead to Robin being branded a demon and forced to live 20 years of subterfuge and lonely misery. What if its that knowledge, and not JUST Roger's power, wealth, and legend that made him such a critical figure?
KNowledge of what happened in that void century could very well make the bearer of that knowledge the most powerful person in the world, for what good or bad that entails.
That's why I believe that One Piece is either 1) The One Piece that pulls together the lost history, the Rio Poneglyph, or 2) The One Piece of history, the whole record of the lost history, amassed into one journal.