This isn’t a major theory of what the hat may be or is in the grand scheme of the plot. This purely is on why and for what reason Shanks had towards it. The thing that immediately stood out, to the point the crew was named after it and his very bounty cites it, was Luffy’s Straw Hat. This was full well intentional by Shanks, perhaps with some shade of knowing fated prophecy parts, but for another particular reason.
They passed the hat on specifically, to be a sign that Luffy was the one he was betting it all on, and that the rest of the Roger Pirates should put their all into helping him out where or when they can. It’s why Crocus would supply them with knowledge and the log pose, and it’s why Rayleigh was both coating their ship and would even fight an admiral for his sake. Because as soon as they can confirm it wasn’t that he stole the hat from Shanks, they confirm that out of the entire world, Luffy’s the one he’s betting on being who Roger was waiting for.
It’s a signal that Shanks never has to once go to anyone else to mention, and that he doesn’t need to elaborate on. It’s a silent message that only the Roger Pirates would catch on to the true meaning of it. And, why he can’t take the straw hat back yet, not until Luffy would be so definitively the one. Only once it was obvious and he wouldn’t need it, could he accept Luffy giving it back. Once he was such a great pirate that they wouldn’t need his seal of endorsement any longer.
*Theory by YayMangos
