The problem lies in how Diablo will escape from the Abyss. My thinking is that the black soulstone IS the Abyss, and the ultimate goal of the player is to destroy the abyss whilst all the evils are trapped within it. How will it happen? Well, watch the first few seconds of the black soulstone cinematic. Leah is flipping through the pages and passes over four very interesting images. One is a serpentine demon, the next is a pentagram with a distorted humanoid figure, the third is diablo's skull, and the fourth is the black soulstone.
I'm assuming that the pentagram image is some sort of ceremony that will be performed on Leah to retrieve Diablo from the abyss and, through the small bit of influence he has on her from indirectly fathering her, allow him to possess her body, which will transform her into the undead looking model that was datamined.
But who is the serpent? My bet is Belial. He's been confirmed to be in the game, but no one knows what he looks like. It wouldn't surprise me if Belial mascarades as an important character that either tricks Leah or forces her into the ritual. At the same time, it wouldn't surprise me if Adria is the one to do it too. She did, after all, sleep with Aidan while he was quite obviously not himself (after all, he did have Diablo's soulstone in his forehead).
The importance of the black soulstone to Azmodan is most likely the same to Diablo if it is infact the Abyss. Possession of the Abyss could give a demonic lord the ability to absorb the power of his brethren trapped within. Obviously being one of only two remaining great evils doesn't make Azmodan a prime evil, so he needs some sort of catalyst, which we know from the trailer is the black soulstone. There's an interesting line in the trailer, however. Azmodan specifically says that he will reign as THE prime evil. THE prime evil is Tathamet.
While speaking to Leah, Azmodan speaks of some sort of trap. It seems possible to me that the trap he's speaking of was a scheme laid out by Diablo to have all the evils but himself trapped within the black soulstone so that he could absorb their powers and become Tathamet, with Leah being the ace up his sleeve to escape from the Abyss himself.
If I'm right, it wouldn't surprise me if we see Tathamet as a boss a la the Uber bosses from Diablo 2.