You know, the more I think about this, the more I realize how brilliant Mashima has been. People have been seriously complaining, and Ive still enjoyed the series, but he's been giving us all kinds of crazy foreshadowing for a long time.
When I have more time, I'll do a breakdown of all the hints, foreshadowing, etc Ive seen since I re-read the series, But for now, there's one really interesting question thats been nagging at me:
Is Mavis also an etherious? If you notice, her body is being preserved in the same kind of crystal as Natsu's was. And she learned some of her most important magic from Zeref, and given how familiar they are, it's pretty obvious their history is much greater than that few days they were together and he taught her some magic. And she also declared herself a black mage, perhaps jokingly. And Lumen Histoire was also once referenced as Fairy Tail's darkness.