User blog comment:B214/Volume 22 spoilers/@comment-2.102.21.54-20160814202124/@comment-95.90.227.125-20160820123402

I don`t think so that Vali becomes a Maou. Its just don`t fit him.

They can change something and so maybe Issei will become a Maou and he can have his Harem. For me the most befitting Titels for Issei would be Issei Lucifer. I don`t know but it would be Nice if he would take Sirzechs Legacy as the new Leader of the Underworld. He can also become the Leader of ExE.