i don't really think this is a plausible theory
The void century revolves around a war between the Great Kingdom, and 20 other kingdoms. Having 20 other kingdoms implies that these are also people who lived on the planet, and resented the great kingdom for the power they held. When the 20 kingdoms defeated the Great Kingdom, they formed the council of kings and the World Government and the celestial dragons are descendants of those original kings.
so it doesn't really make sense to me that the Celestials would be aliens. I do think that Aliens could be part of the secret of the void century, especially with the introduction of the Automata and Enel's cover story, and Sky Island citizens seems to be some kind of alien, but I don't think the celestials are.
Like I said, I think that having aliens in flashbacks would be fine, but I don't really want the strawhats to go into space at all.