Xe’ra makes for a good villain.
She’s not like other Naaru we’ve met. A’dal was benevolent and open-minded, as were most Naaru we encountered in Outland. A’dal didn’t go around Lightforging every demon or Arrakoa that sought shelter in Shattrath. Xe’ra’s zealotry and blind adherence to the Light alone sets her up as a more subtle villain than we’re accustomed to. We saw her build an army simply by convincing people they were in the right, but using circumstances to her advantage.
And we’re seeing it again in AU Draenor, or at least we’re seeing another, ‘Light Mother,’ doing just that again.
The Jailer didn’t exist in the minds of players before we were told he existed when the Shadowlands were unveiled. Xe’ra exists in the minds of the players. We saw for ourselves this domineering Naaru that manipulated people with the Light, that would even go so far as forcing the Light upon them to turn them into weapons for her Army of the Light.
Xe’ra has far more foreshadowing as an enemy than the Jailer ever did. We have more of a personal connection to her, to hate her. For Horde players, she drove the Mag’har out of Draenor. For Alliance players, she toyed with the lives of three of our heroes (Turalyon, Alleria, and Yrel).
Having Xe’ra as a villain will be a far better story than the Jailer, because we’ll at least have some kind of emotional investment in opposing her.
That doesn’t mean the story will be good. One halfway decent ingredient doesn’t make crap soup not taste like crap.