This answer contains knowledge from different parts of the series, especially chapter 416, and up to the current chapter 421. Read it at your own risk.
It has been revealed that Natsu, or another being inside of Natsu which may be dormant, is in fact END. It stands for Etherias Natsu Dragoneel. Crazy connection right? Though after Hiro Mashima tossed that bombshell, he sent the story into another adventure with a disbanded Fairy Tail: guild master gone missing, guild mates scattered across the continent, Natsu just returning from a two year long training by himself in the wilderness (basically pulled a Naruto) and teaming up with a semi-depressed Lucy, who was a reporter by that time, and had been keeping tabs on everyone in the hopes of the guild reforming.
The only thing I can connect Natsu and END is his Dragon Force, but even that's far-fetched since the other Dragon Slayers can also enter Dragon Force. However, from what we have seen so far, none of the other Dragon Slayers have been able to stomach other Dragon Slayers' powers and combine them with their own, except for Natsu, which is where his END part comes in. That's the only connection that I can come up with so far. Gajeel had done it too, so maybe he has something to do with Zeref's demons as well. It would make sense why there are the three dragons that talk together, about the three dragonslayers meeting and them forging their own path from now on.
That and when the series first starts jogging in the "Erza Arc", we can see Natsu ate a rock of Etherias (hint hint) which should have killed him (hint hint) but it didn't. Instead, it just gave him a power up and a massive stomachache (hint hint) in the fight against Jellal who was possessed by a demon of Zeref. The story is now at chapter 421, and those stuffs in the first, second and third arcs are starting to make sense. But as there have been a plethora of arcs since then, you kinda need to go back and re-read it.
Anyway, this is just based off of my observations and what I have connected. If you'd like the refute my findings and opinions in a mature rebuttal you may.