User blog comment:Omega natsu2/Chapter 372: Breach, Predictions./@comment-2214433-20140220145726/@comment-7418318-20140220220148

In another blog (I don't even remember which one...) we specualted that it might be another OVA about one of the future timelines or the time of Zeref.

Some also suggested some more information about Layla's death, but I don't think that'll happen until after (or probably the same time as) the Dragons returned.