User blog comment:Omega natsu2/Chapter 387: Tartarus Arc, Part 4: Father and Son, Predictions./@comment-77.194.131.111-20140604151428/@comment-77.194.131.111-20140604154920

If you look carefully the chapter 386, Mashima two hints for the future : 1. Mard Geer will retrun thanks to the human malice 2. the magic of the stars can clean evil and if you read the GMG arc, Yukino said the 12 keys together can open the door to change the wolrd

It's clear the keys will have a major role to play in this arc, or maybe the following one, they are not just key that open gates, but also talisman which protect their bearer, like they did with Lucy following Alegria