What if they pull another Bolvar where everyone thinks he's dead but in reality he's the Lich King and all. Instead of Illidan being Sargeras' jailor, Velen witnesses Illidan forcing the very remaining essence and power of the Titans to kill Sargeras and end his threat once and for all. He also could use the power to bind Sargeras inside of him using him as his new inner demon without being trapped at the Seat of the Pantheon.