No, not the Dan Brown book..
This is my idea for Diablo 3: Angels and Demons
Deckard Cain: Long have the Angels of Heaven been fighting the Demons of Hell. You, great warrior, like your two predecessors, are greatly needed at this time, at the end of time as we know it. Years ago, a warrior of unsurpassed battle prowess defeated the greater and lesser evils, and he destroyed the soulstone of Mephisto. Diablo and Baal have been... ressurected, by who, I know not, but what I do know, is that a war is brewing between the Angels and Daemons, and that you are the Chosen warrior.
Okay, so the game would be full of the same style of quests and acts, but
1) instead of bodyguards, you meet up with angels, who eventually grow in number, and individually in levels
2)at the end of the last act, you would have all of the angels with you that you met (who didn't die) versus Diablo, Baal, and all of their daemon buddies!!!