Am I the only one here who would be really disappointed if One Piece really WERE some treasure stockpile? The only possible physical thing that I would probably accept as reasonable is for it to be a Poneglyph or something, like One Poneglyph to Rule Them All. Something that clicks everything into perspective.
But I'm shocked at all these people saying they wouldn't accept it being a message about friendship or something, or wouldn't consider the series complete. Quite frankly, the dumbest possible ending I can think of for the show is for the crew to make it to Raftel and be like "hey cool we found One Piece!" That's corny and dumb. The hilarious thing is that the message "it's about the journey, the the destination" is applicable to the reader/viewer just as much as it could be applicable to the characters. For all I care they never get to Raftel.
I'm not sure how many people hold the idea that it's JUST treasure. I think a pretty solid group of us do think it is that final Poneglyph (as someone demonstrated here all the way back in 2005, dang) and possibly some other plot-relevant things at the time. And you could throw in some gold in there, why not, as a bonus.