I feel Belial is likely to be the "new boss" in this game.
At the end of Diablo 2 LoD they destroy the worldstone, which is rumored to allow the hordes of hell to "flood into the human realm".
Being as Belial would be one of the major Overlords in Hell (especially considering Diablo, Baal, and Mephisto were on Earth at the time) I feel he will be the one to march his mighty armys forth into the Human plains.
I hope so, anyway