Okay, so I think a lot of people have seen the new image of Diablo that was shown at Blizzcon. I can't seen to find the image myself, but he looks remarkably more feminine than he used to. His waist is narrow and he has a general shape that leads you to believe that somehow... he's become a woman?
So I was looking at that, and I was thinking about it in-depth, and I remembered that one of the responses to the question of why he looked like that that was answered by a Diablo developer was that we would find out more as we progress in the game... so I had a bit of a thought. What if he looks like that because he has taken over the body OF a woman? In the second game, he takes over the body of the male hero that defeated him in his first incarnation, and he changed to eventually look like his normal form.
My thought is that perhaps he took over the body of Leah from the trailer. Maybe her fascination with the Black Soulstone lead to her being possessed by Diablo so that he could live again?
Anyway, just my thoughts.