So the mass majority -including me- pray that Natsu is not in any way shape or form (SO HELP ME GOD) END. The thing is though all the theories I have heard before don't make sense to me anyway or don't completely add up (YAY). Now I know that in the end, the idea is still possible no matter what (and after me knick-picking all the plausible ideas, it would make it even worse for me), since Hiro has been known to make bold plot moves and leave holes that only end up demeaning his genius, but he is known for other things like not being to overly-predictable. I just hope that this will shed some hope, and actually lead to END not being Natsu.
END possesses Natsu theory
One theory is that END will possess Natsu in the short future. Yeah sounds very plausible, especially since Natsu does not have his scarf. the thing is this would practically have no connection to his name. Why would he be named Etherius Natsu Dragneel, if he isnt even that. He was able to fight Igneel without the possession before (unless you think it happened before but will get to that in the next theory), so any possession would have no connection at all, and may not even be technically a Natsu is END theory. Just "Natsu is possessed somebody save him" theory. Now this can still possibly happen because Natsu does not have his scarf its just this really isn't a Natsu is END theory.
Natsu is REALLY OLD/ Dormant END theory
Okay another theory is that Natsu is very old and END was somehow dormant inside him since his youth or even birth. Igneel had fought Natsu before he trained him or during training. Igneel couldn't defeat Natsu but was able to reseal END. He then leaves Natsu. The thing is though the only hint we have that Natsu is old was from runes back at Fighting Festival arc, and Gajeel was also trapped. Why make him old too? And why did Natsu take forever to age to like 7, but then aged normally from then on. Oh and there's the fact that END is now in a book. Now Igneel could have say ripped the Demon out of Natsu (that sounds painful btw), and END was then sealed into a book. END is now trying to rejoin with his host (this gets to the part of if the possession had happened before). Now again END's name would have nothing to do with it, since he isn't actually Natsu. It would just be a "END is a parasite" theory.
END is a clone
The reason this doesn't work is because END is older than Natsu, and you can't clone something that doesn't exist. If Natsu is in fact REALLY OLD, why clone a child. Could Zeref really see the potential even then? We have yet to see if cloning (and I mean a long lasting, alive clone. not some temporary magic-made one) is possible.
Natsu is a clone
Natsu shows no signs of being a demon, and END has been trapped in a book for quite a while. Again if Natsu is really old, there's still the fact that Natsu is not a demon. Also there's been no sign of cloning being possible.
We know Mashima is not one to over-explain himself, as he leaves himself with holes in the plot, so anything too overcomplicated may (emphasis on may) be overruled like the last three. Also there's a good chance that anything about END has to do with Natsu and his scarf. The other thing is Mashima can be unpredictable. So it may be safe to say that these theories may not come true, and END may not be Natsu or vise versa. In the mean time, we can guess at what else END can stand for.
END will probably some way or another try to cause Natsu to be consumed with darkness, and that his scarf will be found and will protect him before some major cliche- i mean catastrophe can happen. Another idea is that END may try to turn Natsu into a demon, but seeing how Hell's Core is destroyed... My third Idea is that END is a dragon - Etherius N Dragon, but I don't know what the N can stand for.
Also I would like to know if anyone else has some predictions on the connection between END and Natsu.
(srry this was really long :P)