Well i dont see why nomura would bring him back, he wasnt really relevant to the story. He wasnt an apprenctice and he cant be a member of the new organization since xemnas stated he was "weak of body". He cant be a seeker of darkness so there really is no more reason to be involved in the story.
Well, again, we are only assuming that Xemnas called Demyx 'weak of body' he didn't mention his name. But, I digress. He was pretty useless, but still it would be nice to know how he was created, what his life was like and what happened to him afterwards. Well, maybe not. But, I just wish the plotholes of the Organization members of 9-12 got addressed.