I like the theory, think it sounds plausible.
Anyways, i have a little thing i would like to add, regarding who the ancient poeple may be.
look at this cover page,
http://www.mangapanda.com/103-2577-1/one-piece/chapter-470.html
the winged moon people of ancient times. skypians? 2 chapter later the cover story tells us that the ancient city in which these people used to live were named birka. just like the sky island which enel came from. it also states that the ancient people travelled to the blue planet.
at some point i remember someone pointing out that oda told us to keep an eye on the moon.
to me it is at least a clue