User blog comment:FairyRaven/Fairy Tail Chapter 437: Magnolia, Review and Commentary/@comment-24277053-20150525123257

Maybe the 7th guild master is more of a symbolic thing than a literal thing, maybe the 7th master is "all of you", everyone is their own guild master because they all choose their own destiny, thus there is no need for a master, they are all their own guild master.