User blog comment:IamJakuhoRaikoben/Chapter 418: Challenger, Review./@comment-24546844-20150202172212/@comment-24546844-20150202175823

I was thinking of something similar. Maybe during this last year Zeref has been slowly releasing E.N.D.'s power and with this strengtening Natsu. This would explain his massive powerboost. There would be no reason for Zeref take back the book if not for opening it himself right? But he can't just open it, if he could there would be no point in the Tartarus arc, that's why he is doing it slowly. The fact that Natsu wants power so bad to beat Acnologia means that he is accepting this power coming from the book even if passivelly and unaware of it. That's just a theory though. And while he is chanelling E.N.D.'s power he starts chanelling it's personality too to the point that he start losing himself to his demon persona. A bit Bijuu mode or hollowfication.

As for the guild it's like I said, I want Hiro to explore this further than just "Hey Natsu is back let's gather up again" because as I said, that'd mean the guild members don't have individuality. There must be a deeper reason for it to happen, some conflict, the fact that some guys would prefer to keep their new life and things like this. As you said Natsu's transformation is a good reason for it.