Varmithras is suspiciously absent, meaning he has a big role to play later. Sylvanas is being hyped this expansion, meaning I believe a big confrontation with the two former allies is incoming. I think since the Third War he has been slowly turned to the side of the Old Gods, as foreshadowed by Xavius returning to the limelight. He had already been defeated, so why bring him back? Just to put into our heads that Demons (along with the holy dreadlord from the paladin class campaign) don't have to fight for the Legion. Also Chronicle, which is pretty much a Legion tie-in, mentions dreadlords worshipping Old Gods.
Now, we know that after the Legion is defeated, its the Void that becomes our biggest threat. The problem is, Illidan is supposedly destined to become a champion of the Light in the future. But his character, along with his entire Illidari, is built around the SOLE goal of defeating the Burning Legion. Nothing else matters. Something big would have to happen in order to get Illidan to see anything other than Demons as his foe. So, in the final push to slaughter the remaining forces of the Burning Legion, Varimathras will possess Malfurion, (Who was cursed with the possession spell by Xavius while he was kidnapped, why else did Xavius capture him? Revenge?)and Illidan will be forced to fight and kill his brother in order to defeat the last remaining leader of the Burning Legion. Seeing what the forces of the Void Lords are capable of under Varmathras, even forcing his hand in slaying his own brother, gives him enough reason to fight the Void.
Pictures because this got a little off topic