For whoever who doesn't know this, NaLu (NatsuxLucy) has been pretty much confirmed によって Hiro Mashima (creator of Fairy Tail) a while back saying: "Lucy and Natsu's union will give birth to a child named Nashi". Hiro is saying that Natsu and Lucy will have a child named Nashi.
Why NaLu will happen:
Natsu & Lucy represent Haru and Elie who are a couple from Hiro Mashima's 前 manga, Rave. With that 与えられた and the OVA when Elie 発言しました to Lucy that Haru & Natsu are their hero's then that is Hiro saying that it will happen.
Another reason is that if あなた check Hiro's pics on twitter, あなた can see Natsu & Lucy coming up a couple of times.
The last reason is that on a Fairy Tail side story (only manga) there was a time when Juvia is hallucinating and sees Natsu & Lucy doing romantic stuff.
posted by mariojaco
As あなた all know (at least those of あなた who are 読書 the 最近 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 によって 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.