User blog comment:B214/New anime 2017 discussion blog/@comment-97.86.117.132-20161230234256/@comment-9595671-20170102232248

I think what Riser said about bloodlines staying pure is also a reason Rias would have to step down and give the position to Millicas eventually.

It still confuses me, but due to Issei's body even if he was reincarnated as a Devil again, then Ex could have inherited the dragon traits at birth, making him a hybrid. So Ex wouldn't be elligible to succeed his mother.

That's possibly what the Millicas story after DxD ends will be about, a coming of age story for when he eventually succeeds as the Head of the Gremory Household, or atleast it woud be one part of the story.