They can also make it that, we havent killed the real azmodan or baal ..only their shadow
And this time pls dont let the player beat the prime evil all by itself (makes the prime evil lame ), its a "must" to use a quest godly item like in diablo 2 , hell forge hammer ( not sure of the name ).
Or they can make the real Lucifer appear- ( the other side of diablo before he became evil, who separated from diablo as another being ) working together with the missing arc angel of justice on trying to create a new paradise thereby using the 7 lords of hell as pawns to wipe the present heaven and earth. With evil being destroyed the fragile balance is broken so they have a chance to proceed with their plan...So the heroes goal is to of course stop the two nincompoops from totally wiping out heaven and earth. ( oh btw after diablo has been destroyed his essence returns back to lucifer thereby reuniting him with his real good and evil balanced body ( which ofcourse makes him different from other angels blah blah blah..does diablo again exists, this time being more stronger.
Or..in order for Baphomet to be complete it needs the 7 lords of hell + the soul of lucifer..
Id like to see other ideas...i presume more deeper..