HOLY CRAP GUYS!! DESTHWING IS IN GAME ALREADY!!
As you all know, the demon soul was an Instrument the old gods were trying to have created, through deathwing(obviously). He(Neltharion) had the other aspects, red, blue and green to empower it.
Okay, here's how I found him in game.
In the Yogg-Saron fight, one of the visions in his mind is the creation of the demon soul. It's the lower part of wyrmrest temple, and all the flights are empowering the locket. Here's a picture from the fight and DEATHWING is in front of the black dragons. Along with Ysera, Malygos, and alexstraza.
Take a look.
Someone fix my link cause I don't know how to hyperlink it.
http://www.wowwiki.com/Fileragon_Soul.jpg