Well apparently we crushed diablo's body (aiden, and dropped burried him in lava to honor him?) and went back and crushed his soulstone on the hellforge after we vanquished him in d2... Which if I'm correct just leaves him without a body, meaning his soul, being a piece of other living beings (VOLDEMORRTTTTT) returns to the void until it can(chooses to..?) make a connection with something, somewhere. Adria or leah most likely have something to do with diablo's return, intentionally or not. With that said I really doubt/hope diablo doesn't take over leah, though it can't be discounted as a possibility. I like my diablo's meaty and bulky!
If Diablo absorbed the power from all the Great Evils, his feminine legs could come partially from Andariel and not Leah. I still don't think he will posses Leah because he really doesn't have to since there is no more Worldstone, and even the devs kept telling in the panels and interviews that we haven't seen the true form of Diablo - yet. However, something fishy is going to happen with Leah, there's no doubt about that.
And speaking of Andariel and Duriel, when someone asked about them during the Q&A, Metzen said: I wouldn't leave them lying around. They're dangerous! So why not absorb their asses to gain more power.
That conception of the story is so good, I don't understand why people didn't talk about it after your post. And the fact that Diablo use curses from Ball/Mephisto/Duriel/Andariel make it much more believable. And it would explain diablo's new form and the three heads part.