User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-203.215.121.122-20150121234710/@comment-7418318-20150125104339

True, but then we also "know" that the body of an Etherious turns into the book when they are sealed and the other way round and yet here we have Natsu and the Book of E.N.D. co-existing at the same time. If this one thing about Natsu is different from the other Etherious, why not something else too?

But even if he is an artificial life-form, it might still be possible to turn him Human or something closely enough resembling a Human that he might break free from Zeref's control (if Zeref has any control over him to begin with) to live a relatively normal life with his friends just like a true Human would.

I agree that we'll only know once Mashima tells us, but just waiting to see without speculating is just half the fun. ;)