It seems like a reasonable idea that the civilization of Raftel was Atlantis-like and had powers and technology of fantastic quality. And that they are responsible for Devil's Fruits, haki, and the Grand Line.
What would be the point of us? A refined existence for human beings. Recall Van Auger's metaphor, of the Grand Line being a great gear upon which the strongest of the world come together and compete. Life is pretty peaceful in the Blues, but the Grand Line sucks in those at the cusp of natural selection, and is seemingly designed for it to facilitate the cultivation of personal power, innovation, and courage.
Basically, imagine a civilization that has advanced enough that the weights of life became too light to bear. With no challenge presented by the world and the threat of internal despondence, unemployment, a general lack of fulfillment for the species…why not engineer a more vibrant pursuit of happiness?
Think of it like the Matrix: the machines provide a mental occupation for human brains which have been made obsolete. In this case, rather than a fabricated digital reality, the world itself has been altered to allow for some rule changes. Or I guess One Piece could be a more direct Matrix situation in theory really anything could.
For our own civilization, the prospect is not far-fetched. Consider the development of video games, or more specifically MMOs. People flock to alternate realities with simplified struggles, amplified dynamics, flashy circumstances relative to current society and existence. This won't likely stop unless it is made obsolete by some form of escapism even further beyond our ken. Human civilization might be set in motion once again, this time with some of the odds altered, some of the rules re-written, perhaps even detachedly supervised by physically deceased consciousnesses inhabiting an ethereal or somehow permanent and somewhat omnipresent network.
The world of One Piece has some interesting qualities as a potential occupation: allowing greater personal freedom and variability, widening the horizons of human capability whilst preserving the essential challenges of life, to a degree equalizing the power struggle of society.