User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-108.250.20.180-20131117185004

I was thinking they go to the past and Gajeel becomes Acnologia. Which could explain why Levy, a not particularly strong mage, survives. Gajeel/Acnologia doesn't want to kill her if given a choice because of some lingering memories left from the real Gajeel before he was overcome. It could happen they go into the past 400 years ago to the war with the dragons and in the process of taking part in the war Gajeel kills a large number of dragons and is the dragon slayer who becomes Acnologia. Natsu is able to overcome the transformation process and remain human. Flash forward to the present and it becomes not a battle to destroy Acnologia but to save Gajeel.