Hi readers,
I'll make this as simple as possible.
Devil Fruits = Eternal King
- Nika is circulated via the Nika Fruit
- Joyboy was alluded to have failed to be King
One Piece = Eternal Kingdom
- An artifact capable of restoring the Ancient Kingdom's people by using Lineage Factor.
- It also stores knowledge in general: culture, philosophy, etc.,
Will of D = Eternal Will
- It is meant to be shared with the new Ancient Kingdom by the New King.
- The New King was always intended to be a D clan member.
The Eternal Kingdom theory works by combining all three elements to fill in a big picture concept. It is Eternal because as long as all three elements are in-play, then the cycle can continue indefinitely (forever/eternally).
Note: What people need to understand about this is that the One Piece isn't just any item. It's an item connected to an advanced ancient civilization, and it's called a great treasure.
Some people think it's just meant to fix the world that Imu created. For example, they want it to be a terraforming device, but that would imply making decisions unilaterally. Changing all of the seas and the landmasses would alter everyone's way of life. The Eternal Kingdom theory goes beyond that because as long as it exists ANYTHING is possible. That's the entire point of it.
It's very obvious that the Ancient Kingdom isn't as simple as just a destroyed civilization. It is so powerful that it makes every other nation inferior. The World Government tried to erase it and fears it coming back. All signs point to the One Piece being able to do that.