Ok, so I will disregard all of my previous theories concerning E.N.D and this will be my final theory (As well as my prediction for the next chapter "Drop of Flame" as to Zeref might reveal a few things.
Ok, so as to some theories that Natsu could be Zeref's son? I think that is true but I think that there is more to it. I think that E.N.D was not always a demon, but perhaps the decease twin brother of Natsu who died before he was born. Out of grief, Zeref attempted to bring him back to life by turning him into a demon. Even though like the other demons of Zeref were tasked to kill him, he was actually compelled to disobey and did things of his own accord. Zeref could not except E.N.D and turned his attention towards Natsu, out of jealousy E.N.D attempted to kill Natsu. But something happened that may E.N.D be sealed back into a book.
When Zeref created E.N.D. it was his greatest masterpiece, but knowing that he is the strongest caused him to become arrogant and egotistical in his existence forgetting his purpose, at some point he battles Igneel, sensing that E.N.D. became lost in his arrogance, Zeref had no choice but to seeing away with all the magic of Earthland.
Before he did that, Zeref still believed in E.N.D. so he removed an aspect of him and from it born Natsu Dragneel when he was a baby, as a baby Zeref would have him growing up learning the value of life from the world of others.
When Natsu finally faces E.N.D., Nastu must lose in order to defeat him. For when he does lose E.N.D. will finally see the "Value of Life" and thus he will be revealed as the "True" Natsu Dragneel.
Etherios Natsu Dragneel.
I was also thinking that when E.N.D. becomes the true Natsu Dragneel, he would finally become a very powerful dragon slayer enough to defeat both Acnologia and Zeref.