My theory is that Imu and Joy Boy were originally allies.

In the First World, they worked together to build and organize civilization. Joy Boy represented freedom and unity, while Imu represented order and stability.

I believe Imu came from the Moon, as suggested by Birka, Enel's cover story, and the ancient automata. Joy Boy may have helped integrate these lunar people into the world.

At some point, Joy Boy accepted that all things must eventually die and give way to a new era. Imu, however, refused to let civilization end. He chose to remain in power indefinitely and preserve the world under his control.

This created the first ideological split:

• Joy Boy: Death and change are part of life.
• Imu: Civilization must be preserved at all costs.

Later, during the Second World, Imu is still around, and he found out that the world was gonna end. So he choses to save some people and leave everyone else behind. The chosen ones were the 20 traitor kingdoms. Then, a new inheritor of Joy Boy's will rose to challenge Imu's plan. By then, Imu had ruled for centuries and controlled the foundations of what would become the World Government. Joy Boy lost, but left a way to save everyone, hidden around the world with the help of Lili (traitor to Imu).

The Celestial Dragons are the descendants of the people Imu selected to preserve.

Void Century.

The current era is the Third World. Imu still rules in secret, knowing that Joy Boy's will has returned once again through Monkey D. Luffy.

Core Idea

The final conflict of One Piece may not be about good versus evil, but about two opposing beliefs:

• Joy Boy: Freedom belongs to everyone, and all things must eventually change.
• Imu: Order must be maintained forever, even if only a chosen few survive.

In the end, Luffy may prove that no one should be left behind, and that the world must be free to move forward.