Okay, I wasn't really making going a part 2 of this theory in the beginning. However, you guys wrote back saying some interesting comments on the topic and I wanted to reply to them. Speaking of which, before one of you guys write in the review section "Why don't you just send them a private message, YOU IDIOT?!", I'm doing this because I'm too lazy to send each on them a private message and I wanted to let everyone know about it. Also, I'll be put manga spoilers in parentheses. Let's start!
Sam: "I would agree with you on that theory, but really? Natsu having eternal youth? Did you not read the whole manga or at least watched the whole show? I watched that show and it showed his past and most of them looked like he was 10 years old. So that means he can't have eternal youth. Same goes for Gajeel. Either the dragons were trying test them or they met them 400 years ago and they took them to the future. Which is insane!"
First off, I'm not an idiot. I did watch up till the most recent episode of the anime on June 9th, episode 185, and for the manga I stopped reading at the part where (Wendy and Carla destroyed face) because the anime was starting and I'd rather watch the anime instead reading of manga first. I also know that Natsu was child in the flashbacks. However, we do not exactly know how immortality works fairy tail. Also, it is possible that Natsu and Gajeel could have been in the past and brought to present day, thanks to the eclipse project. (For the reason, it would probably be for destorying the END since Igneel couldn't destroy it. Why not place your bets on someone who is related to a very powerful mage? Or it could be what the other user I'm going to be talking about said.) Alright, moving on.
NeonCrescent: "I'm thinking Zeref and Natsu are either of the 3: father and son, brothers, or Natsu could be one of Zeref's demons.
From what you have said, yes it's very plausible they are father and son but couldn't they be siblings too? In their first encounter, Zeref tells Natsu this, "Natsu, you've grown" it's basically something you'd say to a kid you haven't met for a long time so it's pretty vague but this confirms that Zeref has indeed met Natsu when he was a kid.
Another is that I read somewhere...I forgot the exact source but it's pretty reliable...Mashima-sensei told (was it an interview? i forgot) that in the series there would be unexpected siblings. They could be anyone really but I think this is a key point between the relationship of the more important characters in the series like Natsu, Zeref, Mavis, Acnologia, Igneel, END...etc
As for the one I suggest, Natsu being Zeref's demon...in the Tartaros arc; their second actual meeting (Zeref was unconscious most of the time in Tenroujima Arc), Zeref told Nastu that END(Zeref's strongest demon) was someone Igneel could NOT kill (let's recall that Igneel is no ordinary dragon; he's the King of Fire Dragons so he is most definitely not weak) I could only think of two possibilities as of the moment...since there aren't many clues yet...END was stronger than Igneel or Igneel became attached to END (yes, I'm one of the Natsu-is-END-supporters). In addition to this however; when Natsu said he would inherit Igneel's will and destroy END himself, Zeref claimed that Natsu can do it but also reminds him for a second time that END was someone Igneel COULD NOT kill and that Natsu will have to make a choice eventually to either let home live or kill him. And there's also Acnologia, how does he fit in all of this?
The over 80 thing in the Battle of Fairy Tail arc, I am gonna contradict you with that, I think it has something to do with their magic instead...you know, dragon slaying magic being a lost magic? It's over hundreds of years old and since magic is part of the users' body in earthland unlike in edolas, the runes might have assumed Gajeel and Natsu were very old.
If it's the brothers thing we're gonna talk about, wouldn't it be more plausible with Rogue and Gajeel instead? I mean they have the same hair and eye color (the Strauss siblings all had silver hair and blue eyes). Cos I think Rogue would be an important character again later on since Reyos did warn Natsu about Frosche's assumed death in a year.
Ah well, as you said, we won't exactly know anything that would actually happen until Mashima-sensei ends it (which I hope is not soon enough yet)
Thanks for sharing your theory, it was a good read :)"
It was my personal opinion that Zeref should be Natsu's father because Natsu has no father. Sorry for narrowing it down to that. Also, for Natsu and Zeref being brother's, wouldn't that make it plausible for Zeref to be brothers with Gajeel since they have the same hair color. Last thing is that the over 80 thing was another mistake on my part. Another person also commented on this, which was "zorro99".
That's all I got from you guys, so I hope you enjoyed reading this.