User blog comment:B214/Volume 21 info & spoilers/@comment-124.107.216.47-20160616194612/@comment-124.107.216.47-20160624163114

Millicas became the head of House Gremory after Rias retired.

In my personal opinion, i think these characters are going to become Maous in the future. According to Zekram Bael, Maous are the symbols of power in the Underworld.

1. Vali Lucifer ( after the events in Volume 16, he accepted Odin's offer of becoming his adopted son allowing him to be abolished of some of his crimes when he was a member of Khaos Brigade. )

2. Sairaorg Bael ( even Zekram Bael doesn't want him to become the Head of House Bael because he didn't inherit the Power of Destruction )

3. Diehauser Belial ( maybe?)

4. Grayfia Lucifuge ( she even contested Serafall for the position of Leviathan )

5. Gasper Vladi ( look at my explanation at the top :) )