I'm roughly 95% certain that that when everyone finally gets there, it's just going to be a photo album of his crew and his wife, or memoirs of all his adventures, and not actually worth anything. Remember, his exact words were "you will find it in One Piece." So either One Piece is a place, which is possible, or he's describing the state that it's in, i.e. it's intact.

I'm thinking it's just a legacy of who he was. It was valuable to him, but not to anyone else. And all the asshole pirates will be super-pissed and not understand, while guys like Luffy, Jinbei, and Whitebeard, if he had survived, would have gotten it, and understood what Roger was trying to do.

Basically, I don't think there is a treasure, specifically. I think the value is in the adventures Roger has inspired, and in the rebellion against the World Government that the New Era of Piracy has started. His great treasure is memories and freedom.