User blog comment:IamJakuhoRaikoben/Chapter 257: Father's 7 Years, Review./@comment-756329-20111030195909/@comment-4078901-20111030210042

Zeref and Acnologia should appear during the final arc, maybe the Save the World, Find the Dragons Arc. If they appear during the next few chapters and got defeated by Nakama Power, there wouldn't be any strong opponent left.