So far, no major threat has been announced for this expansion yet. However, in a recent interview with JudgeHype, Ion Hazzikostas confirmed that some sort of big bad is inevitable.
There will inevitably be a big bad in the expansion. However, just as we experienced with Mists of Pandaria, we will have to wait to find out who it is. We'll start by learning more about the threats to Dragonflight, just like we did with the Sha and Mogu on MoP. Eventually, we will find out who the threat of Dragonflight is. It will be necessary to explore, understand the world and wait before knowing who is the main threat of the expansion. You have to understand how the story unfolds so as not to imagine that a character comes out of nowhere.While it's entirely possible this big bad could turn out to be a new character — one we will only get to know once we reach the Dragon Isles — there are also several established antagonists directly related to World of Warcraft's dragon lore. While many of them have already been defeated, some may yet still return.
Galakrond — DeadThe very first real threat the Dragon Aspects faced, in a time when they were still proto-dragons, Galakrond was a monstrous proto-dragon and the «progenitor of dragonkind» according to Alexstrasza. Horror struck when Galakrond started to eat the other proto-dragons — which then rose as undead slaves — bound to his will and helping him find more living proto-dragons to devour. Galakrond's insatiable hunger kept growing, and he even began to re-devourer his undead. The Titan Keeper Tyr — greatly disturbed by this threat — decided to stand against Galakrond along with five proto-dragons who we now know as the Aspects. Together, they defeated and killed Galakrond. Tyr,
Read more on wowhead.com