User blog comment:B214/Question and Answers 2/@comment-24098614-20140429212455/@comment-1359322-20140430022417

I doubt Ishibumi will go that far on Issei's story. By the time, Issei can form a team of his own and join Rating Games, Issei would be too powerful already and there's little if not none character development left to do for Issei.

That's why i liked the idea of a Millicas sequel. Well of course Ishibumi might go explore the other religions of DxD Universe by creating a new series with a new protagonist which focuses on a new religion.