User blog comment:IamJakuhoRaikoben/Chapter 383: Surfer Lucy, Review./@comment-24913871-20140511094856/@comment-7418318-20140511095559

As far a I know, Mashima hasn't said anything about that yet, so we could only speculate.

Though it's quite likely that there'll be at least one more arc after Tartaros, dealing with Acnologia and the Dragon Parents I think.