User blog comment:B214/General Discussion 2/@comment-95.210.189.106-20180409165227/@comment-30048084-20180409180843

Roygun is impossible she is banned from normal Rating Games because of the King Piece situation. Her becoming the first Belphegor is unlikely because of the same reason.

Remember the name of Maou is a title so it doesn't have to be someone from Belphegor clan Bennia appears to be very loyal to Sona so I am sceptical about her becoming Issei's second Knight. Bennia could end up in the Harem but beyond that I don't see it happening.