User blog comment:FairyRaven/Fairy Tail Chapter 337: The Golden Plains, Review and Commentary/@comment-5840892-20130628190511/@comment-7418318-20130629135442

Revenge on Fairy Tail for "stealing" the spot of strongest guild in Fiore, or simply taking FT out to regain that status.

And since she lost, despite having all the advantages, she either needs to get more powerfull or weaken FT first, probably involving forbidden magic and/or causing friction between Fairy Tail and their allies.

Or the arc could be taken to a smaller scale with Minerva directing her aggression only at Erza, with the core-team being drawn into the conflict of course.