Such a creative idea!!! Act zero seems like it may be harder to do than what i counter with. What about a single character specific quest in each act? The quest can only be activated while in the game by yourself. Your idea deserves consideration... unless it would cause a delay then the hell with u =).
I think this could work well as a small xpac. If it was implemented like this, No one would have to participate if they didn't wish. This would also give Blizz plenty of time to complete this feature. It would also be neat if they separated the lore sections up for small pieces after each act. It would be a story inside of a story :).
Id like to see the secret greed level actually be in the game /crosses fingers. As long as the secret cow level in D2 but this is more of a cave/dungeon style where you see gold scattered throughout the cave as decoration of course and making your way through and through of hordes of goblins finally reaching the end of this level only to meet General Greed, a hulking sweat ridden goblin ready to die protecting his bountiful chests. I can see him dressed lavishly in gold and jewelry and a crown all shining bright with the gold finish dual-wielding possibly a kings golden scepter and a golden longsword. This General will put up a hard fight but not too hard for the person who has downed many of hells minions.
this is also a pretty awesome idea. All those little treasure goblins opening up their portal to greed's domain. I mean, I pretty much kill hundreds of things just to grab their gold and loot, if I don't deserve a trip to greedland, nobody does.