I have made a break through with a major theory I have for E.N.D. as to how he will be revived from the book.
This also revolves around as to guessing what the Spriggan 12 "Wahru Ihyto" powers are. Considering his name meaning "Bad Person", his epithet "Adjudicator" and his personality involving things that are "sinful" or what ever. I think his powers will revolve around morality and splitting them from his targets. Like Trigon in Teen Titans where he can pull the evil sides from the Teen Titans or in DBZ where Maijin Buu got angry and his evil side is separated.
The point being if I am correct then maybe he will fight Natsu and rip the evil side from Natsu and it will be that evil side that will revive E.N.D., with Igneel's warning not to open the book or destroy it. It will be his evil side to accomplish this.