Ok, this is still all theories until we get closer to the end, but based on some recent facts that I've been thinking about on my drives to and from work I may have at least a partial answer as to what One Piece is. First, let's go over the facts.
1. Robin has been talking about the "True History" but instead has been finding "clue history" and "true history". In each case the history in question refered to an ancient super weapon of some kind lost to time.
2. In SkyPie Robin found a clue stone that lead to a rather nasty ancient super weapon that belonged to the Cydarians (sp?). Although she only found a "clue history" stone it pointed to the weapon's existance. Gol D. Roger had already gotten the real stone and very likely the weapon itself. In Arabasta she found a true history stone that DID tell of a super weapon of old. Two identical stones with the same kind of info on them. Also, she mentioned that other stones all contained about the same thing. IE my point in #1.
3. Gol D. Roger has been to every single place of the "True History" stones and either recovered the stones themselves or the item the stone refers to. I can't 100% prove this fact, but there's a lot of info pointing to this being the case.
This now leads to point #4.
4. With the previous 3 in mind, this would lead me to believe that "One Piece" is one of 5 possible things.
a. A fully assembled ancient "super weapon".
b. A control device for said item a, or item c.
c. A collection of the most powerful ancient super weapons of history.
d. The "True History".
e. Some combination of the above.
What do you guys think? Wouldn't it stand to reason that this may be true??