So with the last couple chapters i have come to believe that acnologia is really natsu i'm sure there is holes to this theory but hey thats why it is a theory. who better to send back in time then natsu. who would use serious amounts of power and still need more to kill zeref natsu would this is how zeref knew who natsu was and why he was so sad he witnessed what natsu turned into. I have more answers but i will wait till another day for those.
new part and new idea
Okay so I made a theory a while back stating how Natsu was the one who went back in time to defeat zeref ie the part above. But the legend says a dragonslayer once sought out the power to defeat zeref and turned into a dragon. I felt like this was Natsu from a different time skip that this happened to. Now here is the twist on my current theory what if igneel was really Natsu and Natsu from the time skip turned into igneel and then igneel found Natsu at a later date to train him for some particuliar reason to change the past properly instead of turning into igneel again.
Now we know that Natsu could not pass through freid's script because of being older than 80 years many have speculated this to be from the muffler around natsu's neck. But how does a dragon have a scarf right. Because it was Natsu's before he turned into igneel.
Now you can say that well how did the other dragonslayer's come about what is not to say they are not like Natsu as well and have turned into dragons by going into the past as well. We don't know how many times future Lucy has been to the past and or changed things. So there could be the possibility she has been trying to stop Natsu from making a mistake.
Other random idea's
Also this could explain how zeref knew Natsu and was sad about him not being at a certain level yet. Also could explain why zeref was on tenrou island maybe fairytail from the future put him there but never knew how to get rid of zeref without creating a horrible chain of events so the stored young zeref in the past.
It could be possible as well that zeref is a dragon slayer and they age differently then other people as the dragons were hundreds of year's old what's not to say that dragonslayer's don't age the same way as dragons
Sorry if I spoil anything for anyone I have been working on this theory for a while.
Something really old I made
After reading the last chapter my assumptions seem more legit. this whole time levy has been writing out the story of fairytail and what has and is gonna occur. i have proof remember when levy found out lucy was writing a novel and said i'm something of a write myself. also the fact when natsu first met and seen zeref he had no idea who zeref was or
how zeref knew him. this was levy writing this part in because iit was important and will be explained later by her. Gajeel had found the graveyard. this tells me that levy got most of her info about the story from gajeel and also lucy's unfinished novel.