Xe'ra no doubt has an equally-zealous counterpart on the AU Draenor.
An expansion could possibly be built around the concept that Yrel and the Lightbound track down the fleeing Mag'har orcs to Azeroth, and decide to convert the rest of us by force as well. Some will betray Azeroth, but the vast majority will resist and fight back.
The Lightbound then retreat to AU Draenor, and we pursue them via Outland. We gather allies there and then, using some space-time mcguffin, map ourselves onto AU Draenor from Outland, taking the Lightbound somewhat by surprise.
We fight them on there turf, which by the way has drastically changed since WoD (as a nice touch,
both garrisons are now smouldering ruins!! ). The final raid would be an assault on Shattrath City, which has been turned into a fortress. We get inside, slay Xe'ra the Lightmother, and that's pretty much that.
Thoughts?