i think that's a treasure property who roger admire most..
maybe his ship or some item he just get from historical stone
i think there's no gold or money in one piece

I think the treasure of One Piece Gol D. Roger buried are the tablets of truth that he discovered.

Remember in chapter 301 when Robin finds the tablet and starts decoding it she finds Gol D. Roger's message that they came and followed the guidance of the tablet and then Robin goes on to explain how there are two types of tablets - one is tablets of the clues, and the other are the tablets of truth - that means that the Pirate King found the clue from the tablet and went and recovered the tablet of the truth.

The theory is that the lost civilization made the tablet of clues and wrote down on them where the weapons of great power can be found and when they follow the clues to it they will find the tablet of truth there in stead of the weapons. This way they ensured that the civilization that came after them won't destroy them because they will want the weapons.

So I think when Luffy or someone else get to the One Piece they will find the tablets of truth that the Pirate King collected instead of any gold there.

i would laugh my head off if one piece turned out to by the carouge, friends and strenght they showed on the jouney.

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Oda made it clear in an interview that one piece is in fact, something of material value

One Piece maybe symbol to become king just like (sword of the stone in story in king Arthur)

Lol, if it really was a one piece swim suit, I would lmao AND then cry out of disappointment! My guess is that 'One Piece' is some mystical scroll which tells the history of the world and stuff. No power can really be gained from obtaining it, only knowledge. But the hidden meaning of getting 'One Piece' might be just to build bonds with your nakama and stuff and work together to get the scroll
Man, I suck at guessing what things really are .

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Simply another normal human being...

I agree with the comments that it has something to do with the ponegliph tablets.
I bet it's the truth that the Marines are hiding from the ppl and using it to rule over ppl. Dragon probably has some hints and clues to it and thats why he's "revolting" against the world government. and then once the crew finds the one piece that explains it all they'll join Dragon to bring down the gov. that's pulled the wool over everyone's eyes for the past century or so.
Of course there might also be materialistic treasures included as well to help out with that "truth". maybe special weapons or something besides gold lol.
thats my theory anyways >3

I personally think it's a piece of paper, with one word on it.
Maybe nakama, or something? xD

When I hear the name 'one piece' I get reminded to luffy's crew not just because they're the main character and all but because they're one tight crew. When they are together they are one piece. Then again Oda is a genius I think it'll be a total surprise on what 'one piece' is really is.

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This.

I SOOO would like to see that happen

Oda knows that whatever stupid idea he gets
is going to work
so it does not really matter

could be a
turtle
and the one piece fanboys would be all
AWESOME
Oda is a genius

So true, it could be anything and the readers would still be happy

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OP is a super sexy one-piece bathing suit.

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.: deviantART :.

As One Piece is as mainstream as you can possibly get, it's probably something like a piece of paper with the word nakama on it, or a letter from Roger saying how important your nakama is. The cheesier the better.

Hmm... I think it's something related to the Void Century and the Will of the D.
It's probably some sort of remnants from the 'once prosperous kingdom'. Though I;m not sure whether it's just the history itself or knowledge or items.