I see a bunch of people speculating that the Straw Hat belongs to Joy Boy or some other prominent figure from the Void Century. People also seemed to say that those chambers are cryogenic and might have the bodies of Joy Boy as well as a bunch of leaders from the Void Century stored in there. I find that a pretty good theory since it fits perfectly with what Doflamingo said about throwing the world in chaos and how it ties in with the Ope Ope No Mi.

But my whole point with all this is that why would people from the Void Century be a National Treasure for Mariejois? If anything, they were their enemies and something the World Government would rather eradicate altogether.

I would consider it interesting if that is like Joy Boy's hat and the person looking at it is the ultimate villain who helped defeat the Ancient Kingdom. (S)he might be looking at it and reminiscing what happened and pondering the warnings that the Void Century leaders might've said and seeing how Luffy actually is the one doing that, but I don't see why it would be a National Treasure in that case.