Hmm... my suggestion:
Prologue: The archangel Tyrael has smashed the
Worldstone, saying that it has been corrupted. However, as it turns
out, Tyrael himself had been corrupted during his imprisonment at
the hands of Duriel in Diablo II Act II. He intentionally corrupted
the Worldstone and misled everyone into thinking that it was Baal
who did it.
Because of the smashed Worldstone, the barrier between hell and
Earth has been broken. In addition, the three Prime Evils Diablo,
Mephisto and Baal have been resurrected. Hell leads an all-out
invasion of earth.
Meanwhile, the hero of Diablo II, The Super Dark Wanderer, picks up
one of the corrupted pieces of the Worldstone and shoves it into
his own head. In the process, he gets corrupted as well.
Act I: Invasion of Earth
Here all the quests take place on earth. You must undertake several
quests to overturn the tide of invasion against earth. Final boss:
The fallen angel Izual, Tyrael's lieutenant, who is the leader of
the assault team.
Act II: The Astral Plane
As it turns out, Mephisto is masterminding the invasion of earth
from the astral plane. You must travel via a portal to the astral
plane and finish off the threat to earth there.
Final boss: Mephisto.
Act III: Purgatory
Baal has invaded purgatory and is bolstering hell's ranks with the
lost souls there. He must be stopped!
Final boss: Baal.
Act IV: Hell
Diablo is busy reclaiming hell for himself. While everyone else is
busy fighting, he is planning to take over hell for himself and
permanently oust his two brothers.
Final boss: Diablo.
Act V (Diablo III Expansion pack): Invasion of
Heaven
As it turns out, the invasion of earth was just a sideshow. The
main event is the invasion of heaven, led by the fallen angel
Tyrael! You must travel to heaven and stop Tyrael's armies at all
cost!
Final boss: Tyrael.
So how? Think I should apply for a job with Blizzard? 
