Luffy’s straw hat was always very important to the story. He was treating it like it was part of his body, maybe even more, like a Nakama. From flashback, we learned that this straw hat once belonged to Gol D. Roger, before he got his name.
That information was amazing. Luffy was now very close to the Pirate King Gol D. Roger, but in chapter 906 something else happened. Back in Mary Geoise, a lone figure (Im) carrying Luffy’s new wanted poster opens the door to a freezing vault. Stepping inside, the vault is revealed to contain a single object: a massive straw hat the size of the figure.
It might predates Roger’s hat by many years, decades or even centuries. So, there is a question: “did Roger knew about Giant Straw Hat?”. I don’t think so. I think that somebody important in Roger’s past gave him the Straw Hat. That person might aware of its meaning or not. But what is the secret behind Straw Hat?
I think that after demise of Great Kingdom there were some survivors. One of them might be a king or leader. He was not able to wear his crown, so instead of that, he was using the Straw Hat. People of importance to his cause would recognize him, but in time his allies would die out and only thing that remained might be the hat, although I don’t think so it’s the same hat as Luffy’s. Someone in Roger’s past was influenced like people in Fishman Island and he start wearing a hat. Maybe he learned about him from the stories, I don’t know. That would fit greatly in Inherited Will. Not object is important, but Idea behind it.
*Theory by stefan999