She will make her own army of the undead. Why? The Aspects are being corrupted, turned against their purposes by madness.
First was Neltharion. I don't actually know what he was supposed to do, but clearly he is no longer doing it. Earth-warding sounds like something dealing with the mountains and dirt, not explosions of rock and lava. The Old Gods corrupted him. Strangely, I see hope for him since they tried to take the Demon Soul from him, so they are now his enemies. With him ignoring their whispers, perhaps he will see his own madness.
We see Malygos now. Once the aspect of magic, he is now fighting it. Is he insane? Perhaps not, but he is clearly failing in his duty. If magic is hurting Azeroth he should fix it, restructure it, not destroy it. His current method could actually turn out to the worse than the reckless use of magic, since shooting pure magical energies out into the Twisting Nether is bound to attract even more enemies. I wonder what his flight does after he dies.
Nozdormu, now this is speculation, but he is possibly the leader of the Infinite Dragonflight. We saw him in the Dragonblight when attempting to reveal the leader of the Infinites. Further, in War of the Ancients he talks about Malygos as if he knows about his future reversal of role and says the same about himself: "Know that I now asssk forgiveness for the wrongs I will cause you in the future, but I must be what I am destined to be... as Malygos is" (p. 712 in the combined books). Will, is Nozdormu already, betraying time?
Ysera is the Dreamer. Her realm is the Emerald Dream. Yet it is turning into a nightmare. I don't think she will remain uncorrupted for long, if she is even now. Already four powerful members of her flight have begun to spread the madness which they experience; we know them as the four green world dragons. Eranikus was also corrupted, though we redeemed him from what I've heard. I suspect that when she and Malfurion hid the Demon Soul where Neltharion could not get it, they put it in the Emerald Dream. The Aspect's spell on it wore off and through it the Old Gods were able to start corrupting the Dream.
Alexstrasza has not yet fallen, as far as we can tell, but I expect something to push her. Perhaps it will be her encounters with the Scourge which... inspire her. She protects life and the Scourge is the current greatest threat to life. Now that Malygos is dead, she can focus on Arthas. Yogg-Saron may give her an idea: what better way to protect life than to make it unending and persist even after death? She will see undeath as the solution and will create her own Scourge. Already it has been shown that the Red Dragonflight, having mastery over life, also has the same knowledge of death.
A plagued dragonflight seems possible. In Scholomance we saw the beginning: plagued whelps. I doubt Alextrazsa would have as much difficulty in transforming her flight to undeath. How long before we adventurers are sent to kill a new set of plagued whelps?
It’s a good day — three new soundtracks!
6 hours ago