Quote from paralleluniverse
Imperious is not corrupted. That's just his usual personality.
I didn't like how Diablo seemed to have killed Imperious and then he suddenly shows up at the door to the Crystal Arch as if nothing ever happened.
What happened to Leah was sad and a great tragedy. Bringing her back will just cheapen that.
The next expansion should focus on Malthael and his disappearance. An idea could be, after disappearing for 20 years to study the significance of the destruction of the worldstone, he concludes that the universe no longer has a point and so he wants to destroy all creation.
Another potential idea is to explore the power vacuum in hell left by Diablo's defeat. The idea of Inarius escaping his prison and coming back to reclaim his Sanctuary would also be cool.
I agree the lore must shift from the Prime and Lesser evils being the "baddies" to Malthael or even Imperius not corrupted by diablo but just wanting to end Sanctuary and all mortal life as he intended to do because Sanctuary is seen as an abomination in his eye's.