User blog comment:FairyRaven/Fairy Tail Chapter 399: Wings of Despair, Review and Commentary/@comment-24066070-20140908130106/@comment-24066070-20140908140558

Acnologia feeling threatened by all these Faces was indeed what crossed my mind.

Your idea is very interesting as well.

Perhaps the Faces, or even Lumen Histoire, could reverse Acnologia's transformation and Marde would then somehow transfer all of Acnologia's power to E.N.D.

It could be nice if Acnologia is left powerless afterwards, but decides to train Natsu and the other first-gens on how to use their DS powers and Dragon Force so that they can stand up to E.N.D. and Zeref.

After all, most main chars have someone train them: Naruto had Jiraiya, Ichigo had Yoruichi, the Visored and Zangetsu, Goku had King Kai, etc etc.