User blog comment:B214/Question and Answers 6/@comment-90.202.240.151-20161009210010/@comment-29963364-20161011015711

I know Ishibumi said that DxD G might be Issei's last power-up (at least in this series, since we know he gets stronger by the time EX happens). But if he were to get one final power-up, it would be for him to dragonify his body, not just an arm. This might actually be the key to him being able to handle the full-powered DxD G without the side-effects.

His new body is made from Great Red's flesh. He should learn to use it before the series ends.