What is One Piece?

[poopdeville]

I bet it's One Piece of Gold. Probably a Bellie coin.

That and the Rio Poneglyph. I've got a feeling that the government kicked the crap out of everybody with D. in their name during that hidden history period. Roger probably found the Rio Poneglyph and told everyone to find One Piece to get the strongest people with D. in their names to find it, discover the truth, and topple the government.

[DarthEnderX]

As for my opinion as to what OP is, I agree that it most likely is something symbolic...kinda like "One Thing" the secret of life is 'One Thing' in that if you focus on your goal and then achieve it (e.g. getting to the end of the Grand Line and becoming Pirate King) then you can die happily b/c you fulfilled the goal that you set out to.

So, Oda based One Piece on City Slickers?

[ven0m]

Might be something to reverse the effects of a devils-fruit?

[DarthEnderX]

What kinda treasure is that?

Nobody on the crew strikes me as the kind of person who misses being able to swim all that much.

[Death BOO Z]

Maybe it's a real big button that says "push me"...
and when you push it you change the location of the grandline islands, and by that you create a new pirating era, since things have just been shuffeled up...

maybe its' a compass/map that actually works in the grand line, and allows you to get anywhere in there without relying on the silly compass they use now.