Both are very good theories, I haven't fully came up with my own yet but I believe that Deckard Cain (unintentionally) has something to do with Diablo's return. In the Cinematic Teaser, it shows Cain "not feeling well" after he "returns to the ruins of Tristram's Cathedral seeking clues to new stirrings of evil."

So I think that either Cain gets possessed (somewhat like in Diablo II where the hero from Diablo I gets possessed) or maybe Cain is just getting old and dieing.

Well, that's my 2 cents.