User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-71.222.52.171-20151210023306/@comment-4829046-20151210164422

Well Mashima already brought in 'physical' Gods that can be destroyed, so anything is possible really. Already we've had many things happen which had been predicted long ago and were deemed by many to be too ridiculous to go beyond fan-theory back then. Not that we want a Kaguya situation but this is Mashima we are talking here. As long as Natsu survives at the end, anything can go.

If you think about it, even if Ankhseram doesn't manifest or something, the only way to wipe out Zeref's adverse influence is to tackle it at the root. Even Mavis's condition was a result of "Zeref's" meddling as far as Ankh is concerned. There might even be an element where the curse can only disappear when the forbidden act a.k.a. bringing back Natsu Dragneel, is negated entirely. That or, we need another contradictory moment via Fairy Heart that somehow overwrites Zeref's magic.