We are fighting against Primal Incarnate Dragons...so is it correct to say that we are going to fight against 4 deathwings?
Under the old god's corruption, Deathwing's body was out of control, more elemental. Now we have Primal Incarnates embrace these elemental features and they seem under control over those elemental bodies.
Considering the Old Gods like to use elementals and flesh beings to do their bidding, I would be surprised if these Primal Incarnates don't have any relationship with the Old Gods.
On the other hand, if they have any relationship with the Old Gods, Blizzard is introducing the biggest plot hole. The main reason is the Old Gods had the perfect weapon to conquer all Azeroth, 4 elemental dragons with full power. The Watchers and Aspects weren't able to defeat them, only trap them. I speculate if this happens, then Galakrond mutations were a consequence of the Old Gods manipulations.
I hope the design of these dragons are extremely cool.