How much you want to bet the name of the island that the old archeologist was about to say before he was shot is....
(scroll down, unless you don't want a possible spoiler)
and Gol D. Rogers one piece is the Rio poneglyph with the true history on it that Robin's been searching for... and Roger sacrificed himself to draw attention so that a force (pirates) would defy the world government to find this "one piece" and be able to expose the true history to the world.
Actually I also believe that Roger is Luffy's gradfather, the grandfather Aokiji mentioned being troublesome, but maybe Roger couldn't even beat Aokiji, and so his death wasn't really a sacrifice for him, but his last words were for the same purpose as stated before. Wealth, power, fame? He says he had this, which may make you think that one piece is wealth, power, and the fame that comes with it, but maybe he said that also just to get pirates to think that's what they're going after.
If it ends up that the name of the island is not Rio, then it probably begins with the letter D.