I was talking this over with a friend recently and I thought it was interesting. He pointed out how the Xaela's holy relics were actually allagan in origin, which calls to mind how the Ixal's holy land was actually Azys Lla.
In the Aetherochemical Research Facility there is this plaque that talks about them trying to artificially create a voidsent and that "future research look promising." So maybe the Au Ra are the result of them trying to combine a hyur/elezen/whatever with a voidsent?
If that's the case, it makes me wonder what the deal with the dusk mother and dawn father in Au Ra creation myth really is.