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.
This seems to have come to pass, in a way. Bolvar was saved by the fires of the Red Dragonflight and persists as something which seems between life and undeath. She seemed to have a plan for him.
They must not discover the fate of the young paladin. Not yet.
With the defeat of Arthas, Bolvar takes up the crown to become the new Lich King. He controls it, or at least attempts to. Could the red flight control him in turn? He may be the path by which Alextrasza has her Scourge.
This may not be the end of the story of her and the undead. While Wrath of the Lich King is ending, we know what comes next: the reemergence of the aspect of the earth: Neltharion... Deathwing. I do not know what research he did into undeath, but Nefarian, the current (and thought dead) leader of the black flight had necromantic powers and was experimenting with different breeds of dragons. The crossover into a undead dragon breed is not unimaginable.
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?
I was wrong predicting a red plagued flight, but I think we just might see a black plagued flight. Again the life vs. undeath battle will come, and Alexstrasza will have the Scourge at her command. What will she do with it?
On a related note, I will pretend I was brilliant for predicting we'd end up fighting the Titans. Or I would have been if we'd all wiped on Algalon.