User blog comment:B214/Volume 22 spoilers/@comment-82.50.121.73-20161206231912/@comment-29963364-20161207002046

I don't think that would be a Sub-Species, but an Abyss-Side.

* Sub-Species is where the Sacred Gear takes on a different form depending on the user's personality and/or unique characteristics.

Example(s):


 * Cao Cao's Sub-Species Balance Breaker [Polar Night Longinus Chakravartin] was created to suit his technique-type fighting style.


 * Sairaorg Bael's Sub-Species Balance Breaker [Regulus Rey Leather Rex] was created to suit his "straight forward", hand-to-hand combat style.

* Abyss-Side is where the user trains and polishes their Sacred Gear to absolute perfection, and still try to go further.

Example(s):


 * Tobio Ikuse's Abyss-Side Balance Breaker [Perfectus Tenebrae Lykaon et Fortis Densis Laelaps] was a result of him training with his already awakened Balance Breaker untill he actually "bacame one" with his Sacred Gear (turning into a massive, humanoid wolf-man wielding a sythe).

So if Issei were to Dragonify his whole body, and then fuse/merge his DxD G armor with himself, that would be closer to an Abyss-Side than a Sub-Species.