With Fairy Tail coming back tomorrow, I decided to make a fanfiction where I will explain a theory that I strongly support. As you can tell from the title, it is how Zeref is connected to Natsu. By the way, I might be spoiling the manga depending on what I talk about. If you have not read the manga, please ignore the words that are in parentheses.
Let's start at the very end of the Edolas arc., when Zeref actually appeared on screen. It was at the part where his magic accidently killed some wolfs and plants. He was so sad that he continues killing living things and says that he wants Natsu to kill him. That part made me think for a minute. Why did he specifically want Natsu? The rumors did not say his actual name, they used Salamander. However, he did say his real name. Also, the rumors said that Erza and Laxus were stronger. If he had just heard the rumors, he would have wanted Erza, not Natsu, since she was able to take a full blast of the Jupiter cannon AND fought the strongest of the Element 4. This means that Zeref at least knew Natsu personally and saw that he might be strong enough to kill him.
One small thing I'd like to add before I talk about another point is that in the Tenroujima arc., Natsu had no idea who Zeref was, so that concludes that he might have been really young when Zeref met him.
While the first point proved that Zeref knew Natsu personally, the next is going to tell us how he knew him personally. Ahem. In Fairy Tail, Natsu said that he doesn't remember his actual father but only remembers his foster father, Igneel. Zeref tried to keep himself isolated, so he wouldn't kill anyone. Could it be possible that Zeref abandoned Natsu when he was really young, so his powers wouldn't kill his own son? He would've wanted him to get stronger to kill him. Zeref is the most likely character to be his father unless Natsu's mother was any of Gildarts girlfriends. However, there is something people say that makes it impossible for Zeref to be Natsu father and I'll explain why that statement is wrong in the next point.
Alright, the next point that will be talking about very complicated by the way. It's about Natsu's age. People say that it's impossible that Zeref is Natsu's father because Zeref is too old. However, Natsu's age hasn't been confirmed at all. (In the manga, it says that Natsu's age is UNKNOWN!) Also, Zeref has been living for over 700 years and he still looks like he's in his 20s. Natsu could too have eternal youth if he was Zeref's son. Some actual evidence from the story is in the 'Fairy Wars arc.'. Along with the girls turn into stone and the Guild Master, Natsu was trapped in the guild from the barrier. The rule was exactly word for word like this:
"Those's over 80 or stone statues may not pass."
Could Natsu be over 80 years old? However, Gajeel wasn't able to pass through the barrier. This could mean that Natsu wasn't able to pass because it also effected dragon slayers for some reason. This is only proved since Gajeel was there but could it mean that Gajeel is Natsu's brother instead? Since Wendy or any other dragom slayers were not there it prove the first suggestion also, there is a possibility that they are both over 80. Zeref could have twin sons. However, he only said that he wanted Natsu to kill him. Maybe Gajeel is connected to this somehow? Then again, all dragon slayers are connected in a way. We don't exactly know until Hiro Mashima actually adds it in the end. Anyways, I am so looking forward to the continuation of the anime! That's all for me now!