Leah (Cain's daughter) foresees future events and has troubles to believe in her psychic power intially. She has premonitions about the meteorite which shortly after hits the New Tristram Cathedral and falls into its depths. This will reinforce Lea's beliefs that she can definitely foresee future events and pushes her to warn the people dispersed across the World of Sanctuary of the impending invasion by the Burning Hells.
Angel Imperius will attempt to bring his agenda from the Sin Wars to fruition - the destruction of the World of Sanctuary (which he believes is the result of Angel Inarius's corruption) - by conditionally ressurecting the Prime Evils - Diablo, Mephisto and Baal.
As the story unfolds, the Prime Evils recognise that they do not stand a chance to destroy the World of Sanctuary individually (since all the tribes/ clans gathered forces after Leah's forewarnings). They thus decide to combine their forces by fusing into one (hence explaining the three-headed demon, Diablo 3.0 if you will).
It will therefore be the heroes's destiny to initially prevent Imperius from ressurecting the Prime Evils (after Tyrael fails to do so). The heroes manage to kill Imperius at the end of Act III but fail to stop him from ressurecting the prime evils. They will eventually have to slaughter the new shiny Diablo in the final act to prevent the World of Sanctuary from being annihilated.
Just an idea. Maybe it is heaven vs hell, in some strange way Diablo is resurrected and Tyrael will fight against him. Then, the hero get to choose whether to help Tyrael and purge the land of evil or help Diablo and dominate Sanctuary and bring chaos to the gates of heaven.
Here's my idea: after destroying the worldstone, it'd destructive aura (thank's to Baal) corrupts Tyrael, and he decides to revive the prime evils, and has therefore become thier leader, and the 7th lord oh Hell. I think you know where the story goes from there ;).
this topic is pretty old way to bring an old topic and Nachoman i agree with you except reviving the prime evils part because the rest you fight in Diablo 3 is the rest of hell so i mean i agree i think Tyrael will be the last boss but they should bring him in an expansion.