This is my guess as to what the one piece is

What is the one piece?

• It must satisfy all these criterions:
  • A physical object
  • From void century
  • Funny
  • Reward for Luffy(from Oda)
  • Will ‘Flip the world on its head’ (the wording is very important)
  • Roger was too early
  • "一つなぎの大秘宝" (Hito-tsunagi no Dai-hihō)
    • Connects People
    • Makes the seas 1
    • A single piece of string or rope

The One Piece is a Haki Knot woven from the single red string from Joy Boy's Straw Hat. Left behind 800 years ago, this knot contains all of Joy Boy's prime, planet-shattering Conqueror's Haki. Because it is a divine seal of willpower, it can only be untied by one specific frequency: the rhythmic "Drums of Liberation" heartbeat of the awakened Sun God, Nika.

This simple red string is the ultimate ancestral thread of the series, physically connecting the wills of Joy Boy, Gol D. Roger, Red-Haired Shanks, and Monkey D. Luffy across generations. When Roger conquered the Grand Line, he burst into tears of laughter because the ultimate, world-ending weapon was wrapped up like a humble piece of hat twine—and he realized he was too early to open it.

When Luffy finally unties this knot at Red Line, the sudden, catastrophic release of Joy Boy’s stored Haki will act as a planetary shockwave. It will completely shatter the Red Line—which is itself just a colossal, unnatural red band choking the earth. As the continental wall crumbles, Princess Shirahoshi will command the Sea Kings to lift the global ark, Noah, safely carrying the oppressed Fish-men up to the sunlit surface. Simultaneously, the tyrannical elites of Mariejois will crash down into the ocean depths. By destroying the borders of the world and plunging its corrupt rulers into the sea, this single red string will literally and poetically flip the entire world's hierarchy completely on its head.