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The Diablo III site has been updated with a new developer post to discuss 'adventure mode' that will arrive with the Reaper of Souls expansion. Adventure Mode is a separate entity from Campaign Mode in that all story elements have been removed and all waypoints are unlocked for players to explore where they wish. In addition, various "bounties" will randomly appear on the map for further fun and adventure.
Lastly, Nephalim Rifts are profiled:
Nephalem Rifts:Nephalem Rifts (previously called "Loot Runs") are randomized dungeons that are designed to be completed within 10-15 minutes. The goal of Nephalem Rifts is to provide players with a rewarding, endlessly replayable experience, one that continually offers new and exciting moment-to-moment gameplay. . .as well as heaps of loot. They're also what senior level designer Jesse McCree likes to describe as an opportunity to "break all the rules."
So, what does this mean in terms of actual gameplay?
First off, it means that Nephalem Rifts are completely randomized—random interior and exterior tile sets, random layouts, random lighting and weather, random monsters, and (of course) a random boss encounter to cap it all off. Each Nephalem Rift dungeon can be up to 10 levels deep, but even that's random too!
Read more on the Diablo III site.