User blog comment:B214/Volume 21 info & spoilers/@comment-199.216.41.253-20160419203901/@comment-24.113.129.70-20160419233807

DxD Mode is just Ophis' power. Great Red's power has yet to be unlocked. I think that when Issei does unlock it, DxD Mode will be complete and then it will truly be his Final Form.

Don't know how Ishibumi will do it, but I bet he'll find a way to make Issei attain it by the end of the series. Maybe in the arc after the next.