I don't know if this is an original theory or if someone else has already suggested it before, but I thought it was an interesting one to share. I tried to search for it before, but didn't find anything close. Apologies if you've heard it before and my google kung-fu is just weak today.
The One Piece is the unification of all Pieces of Eight, and is capable of a supernatural ability (e.g., granting a wish, as in Dragon Ball Z, which like One Piece is inspired by Journey to the West, but maybe not as reality-altering as that).
It has to be something powerful, though. Otherwise, why would anyone covet it? Why would Whitebeard make it a point to announce that the One Piece is real if it wasn't an artifact of legendary power?
Pieces of eight is a common trope in pirate stuff, based on the Spanish dollar.
My thoughts here are, there were eight pieces of treasure that could be reunited, say, every 4 years (i.e., the Levely time period), to accomplish something grand.
Here, my mind circles back to wish-granting, but it could be as simple as "create a devil fruit out of one's dreams". We have recent evidence for that being a real thing with the Egghead Island arc.
However, Gol D. Roger made off with all of the pieces of eight and kept it together on Laugh Tale, as One Piece, rather than using it himself. Thus, they have been absent from the world government's hands since his reign.
This implies that he and his crew had to have acquired all eight pieces. Some were probably from rival pirates (Rocks), but others were probably from marines or celestial dragons.
As famous as Roger is, we don't actually know a whole lot about what his crew actually did, aside from winning a few fights but not killing their opponent.
I dunno. Maybe that's where the eight pieces were created? Maybe Joy Boy was involved somehow. (Did he steal it from some ancient tech on this island, perhaps?)
Something deeply ironic is probably involved.
If we assume they scatter like the Dragon Balls in the other anime, then obviously no. He left it all on that island.
If we assume they just deactivate and collect power, possibly from absorbing sunlight, over time? Possibly.
He could have wished for everyone to be "free", and then history is taking its necessary course to ensure that happens? But then again, he managed to start the great pirate age without any overt reality-warping being necessary.
I've been pondering Shanks' ability to overpower future sight haki, his own ability to observe the future (i.e., with Kidd), and his incredible foresight in general.
Future sight haki seems to work over very short time periods. Not nearly long enough to plan a sea voyage to, say, intercept a Yonko or an Admiral to prevent a fight. He didn't see Kidd's attack until right before he began to form it.
There's also his apparent admission to the Gorosei's chambers and statements about "a certain pirate", right before the necessary steps were taken to awaken Gear 5 in Luffy.
Although it's possible this is a red herring (lol, "red hairing") and he was talking about Teach.
I think, like many other theories, that time travel is involved somehow, but with Shanks in particular. Or maybe his future sight haki is just that overpowered.
But I think the theme of "inherited will" is more pertinent to the endgame of the story than making power-scaling difficult. :P
Anyway, those are just my theories. Hope you enjoyed reading them.
(Note: If any YouTuber decides to cover this theory, make sure you include my avatar when crediting me. ;P)