Re: What is One Piece?!?!
I think we agree that at the end of One Piece, everyone from the crew will have obtain their dream.
Let's have a look at their dreams and the mean to obtain it :
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Lufy : Becoming Pirate King : Just get to Raftel and find One Piece (is that sufficient to be the PK?)
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Zoro : Becoming Strongest Swordman : Beat Mihawk
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Nami : Drawing a map of the world : Adventure the Grand Line
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Usopp : Becoming a brave warrior : Adventure the Grand Line
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Sanji : Find All-Blue : ... well, find All-Blue

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Chopper : Find a cure-all treatment, the Panacea : ??? (see note below)
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Robin : Discover the history of the Void : Find the Rio Poneglyph
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Franky : Sail in the Thousand Sunny : Adventure in the Grand Line
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Brooke : Get back to Laboon from the other side : Adventure the Grand Line
Note : I'm not sure it's really Chopper's goal, but he talked about it, and I think it would be a great dream for him to accomplish, so I put it there anyway
As you can see, many of those dreams can be done only by adventuring the Grand line, and some are unrelated to the conquest of the One Piece (ex : Zoro beating Mihawk).
Considering that finding One Piece will most likely be one of the last thing that will happen in the manga, I think that finding it should solve most of the dreams of the crew.
For example :
-Maybe the Rio Poneglyph is on Raftel.
-Maybe the All-Blue is around Raftel.
-Maybe the Panecea can be found on Raftel.
But, most likely, story-wise, I think that One Piece is knowledge of what happened in the century of void, and the truth about the Will of the D..