Maghda states in Act 1 when you're finding the pieces of Tyraels sword in one of the rooms addressing Leah that "Your mother shares your fate" (or something along those lines, I don't remember it word for word) Leah's fate is obviously to be possessed by Diablo which is hinted a plethora of times up to the end of Act 3 and actually happens then. I think anything other than Adria returning as Diablo for the expansion would be a shock.