User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-16281343-20150310114346/@comment-16281343-20150310175530

Nakiri Ouryuu is already part of the new Student Council. Him, along with Milaca Voldenbergh, are basically new characters with little to none information. So, it will only be a matter of time Ishibumi makes them participate either in a short story or the main Volume.

Other than Ouryuu, Akeno's cousin Suzaku, imo has the best chance of appearing in DxD. Maybe she will when the focus once again rotates to her.