The One Piece will be, a 1 cent piece. I’ll explain, One Piece is inspired by Treasure Island and in that story the main money/treasure was called pieces of 8, a single coin cut into 8 pieces. So currency of the ancient kingdom in One Piece being called pieces, is likely, and 1 cent could then be called one piece. This is why Roger laughed when the world called his treasure the one piece:
That explains the name of the treasure, but there are actually many more layers to it, Oda has positioned the original Nika and Roger as jokester like Luffy. The idea of great treasure turning out to be 1 cent falls in line very well with what you would expect from a trickster god and what Roger would laugh at.
Oda himself is also a jokester who likes to hide messages in plain sight. Supposedly there is a secret hidden in the title of One Piece:
Looking at the way Oda always writes the One Piece title and his signature for fans, he always puts an X marks the spot over the O in One, the O is probably the hint:
The O looks like a Coin and is probably Oda’s way of joking around every time he signs his name, secretly hinting at the One Piece.
Oda also said the One Piece is a real treasure, not just the journey along the way. 1 cent would be technically a real treasure, even though it’s not worth hardly anything, enabling Oda to make his claim true while trolling everyone with the real thing of value being the journey.
Now before you get mad and click off this thread though, I told you this has many layers, and while 1 cent is basically worthless in the conventional sense, it has huge worth in the One Piece world in multiple ways,
1. The Davy Back Fight
If you haven’t seen my theory before that Davy Jones was then original Nika, go check out my thread about Luffy’s real dream in my signature. That thread also explains the importance of the Davy Back Fight to the end of One Piece.
For this thread though the importance of the coin will be that it can be used to initiate a 1 Coin Davy Back game winner take all against IMU:
2. Proof of the Ancient Kingdom
The coin will have the name of the ancient kingdom on it, serving as evidence of its existence and we know the name is important as clover was killed to avoid it being revealed. Luffy revealing the coin to the world as One Piece would be a huge troll moment to the world, but also serve as revealing the name of the ancient kingdom to the world.
3. The Monkey King
The coin will also have a picture of a Monkey wearing a Straw-Hat as a King on the back of it. Alongside the poneglyphe explaining that this is the future King of the world and his currency. Something Roger would also laugh at serve to explain some of the prophetic things the Roger Pirates knew.