Just a theory based on some things we've encountered in game, also based on the lore given.
In WoD when we kill Cho'Gall in mythic Highmaul he mentioned his "master" is coming. If it is to be believed he was already down the path of the Old Gods/Void as the main universe counterpart, I figured it was the same Void Lords as our main universe.
For creatures/beings/entities that are powerful enough to scare Sargeras (well if not them directly, then the ability to create a Dark Titan for him to be directly scared of) I find it hard to believe the Burning Legion is the only force in the great universe - the same universe which saw the Light and the Void as it's first creations - to transcend all realities.