Okay, I don't have a lot of time and I just wanted to share this to other people, so here I go.
Is anybody other than me thinks that Zeref might personally know Igneel? If I remember right, there was an episode during the arc with the Raijinshuu and Laxus trying to take over the guild, that Grandine and Igneel were talking about Zeref, though Grandine stopped talking as she kind of felt like Igneel didn't want to talk about him.
In the Tenroujima arc, Zeref was shown and he seemed overjoyed seeing Natsu, and it seemed like that wasn't the first time and he looked shock when Natsu didn't recognise him, so I think that Zeref might also be another kid that Igneel adopted and took in, and seeing that he's lived for a long time already, maybe he was also a dragon transformed to a human.
Maybe Igneel started detesting him because of what he's doing to others, I think it's also possible that Natsu was teleported to another world and that the world of the Dragons are different to the world where Fairy Tail is in and that the Dragon Slayers were just somehow transported to that world, and maybe the Dragon Slayers were sent to that world to stop Zeref.
Acnologia might just be one of his pawns.
Okay. I feel better. Bye :D