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 EarthHere 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 PlaneAs 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: PurgatoryBaal has invaded purgatory and is bolstering hell's ranks with the lost souls there. He must be stopped!
Final boss: Baal.
Act IV: HellDiablo 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 HeavenAs 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?