So with the last couple chapters i have come to believe that acnologia is really natsu i'm sure there is holes to this theory but hey thats why it is a theory. who better to send back in time then natsu. who would use serious amounts of power and still need more to kill zeref natsu would this is how zeref knew who natsu was and why he was so sad he witnessed what natsu turned into. I have more answers but i will wait till another day for those.
spoilers as well.