As te all know (at least those of te who are Leggere the recente FT chapters) Natsu will die if he kills Zeref -which he needs to, to prevent the world from ending-. The reason Natsu will die, is because Zeref created Natsu as a demon in the book of Zeref. So if Natsu's creator dies, so does he. I believe that if Natsu becomes human, he won't die, because he will be seperated from his bond with the book of Zeref. And in my opinion, there is only one way to make Natsu a human. LUCY. Natsu will become human if he is loved da a human. He will create a different bond with lucy and break his old bond with the book of Zeref. That way, Natsu can sucessfully end Zeref, save the guild and the world! In conclusion, i believe that Lucy is the only one that can save Natsu's life.