You've been warned
The meteor is Tyrael, he fell from the Heaven because he renounced his "angelhood" by ripping out his wings. We rescue him just after the Skeleton King and from there on he traveled with the heroes.
Cain dies at the end of Act 1 from wounds he received earlier and Leah decides to continue his work.
Then we go to Caldeum and find Adria who has the Black Soulstone and we defeat Belial and trap his soul in the Black Soulstone.
Then Leah has a vision of Azmodan invading Sanctuary from the remain of Mount Arreat, so we travel to the Bastion Keep and defeat Azmodan and also trap his soul in the Black Soulstone.
So the Black Soulstone now contained all the Prime and Lesser Evils, it was then that Adria betrayed us and used the Black Soulstone on Leah to awaken Diablo as the Prime Evil, from the seed he had left in Leah.
Diablo attacks the High Heaven killing Imperius and wrecking the place. We rescue Auriel and Itherael and go on to defeat Diablo.
Tyrael takes the place of the archangel of wisdom as a mortal. The End?
So what's left? Obviously Mathael is M.I.A. and there may be a connection between Adria and Lilith.
Beside those two, who could appear in the expansion?
Metzen also hinted that he may want to "redeem" Leah. I hope not we don't need to repeat SC2's story (which was lame).