Blizzard is putting much more emphasis on letting the player explore the world of Sanctuary this time around. Diablo III will have four acts, so that means we should reasonably expect to explore four habitats along with their dungeons. I know that while we play the game, random quests and caves will generate along the way. I think it would be cool if Blizzard made it extremely rare for an 'Underground Passage' type tunnel to spawn, and lead us to a zone we wouldn't normally get to explore. Lets say they let us see Ureh in act II, but a rare zone might spawn that leads the hero to the Swamp Land, to find a golden treasure chest or fight a boss... or both. Making this event extremely rare will reward the dedicated players that explore all the nooks and caves of Sanctuary along with providing extra content for replayability. ... and achievements!
Here is how my end game MF will be
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Me: act1
friend_1: act2
friend_2: act3
friend_3: act4
*All of us search for the "special" dungeon. It may possibly glow in reddish color.
Once we find the dungeon, we gather up and enter
If the dungeon does not exist, we exit and create a new game.
Sounds similar? Diablo 2 all over again!