User blog comment:Stbk22/Possible power up/Techniques 2 for New Arc/@comment-24.113.252.213-20160805044822

The only power-up that I can imagine for Issei (that makes sense in the context of what we've seen in the series so far), is him learning how to fully dagonify his body (like he did to his arm in vol. 14) and mastering it so that he can use and undo it without Akeno or Rias' help. This way his base stats are increased immensely, which will enhance his Sacred Gear's capabilities (Issei is always given crap about his weakness holding Ddraig back), and he'll probably be able to use the DxD Mode to it's fullest potential.