Blizzard's currently testing Diablo III's first expansion Reaper of Souls on their public test realm, at least in some capacity. Given that modern nerds are wonderfully talented at sleuthing through data files of any type, it's only natural that Diablo III fans have mined the heck out of the new PTR client and posted their treasures online. The fruits of their labors include several new features, story, setting and character details, along with plenty more.
It's worth going ahead and noting a few things here. One, everything on DiabloFans' list of datamined content should be considered: spoilers, consider yourselves warned; likely to change, as the game's development is still a ways out and Blizzard's certain to be very careful with Reaper of Souls; finally, there's the chance that some of this information is 100% inaccurate or full of errors, as junk data often gets pushed to PTR servers on accident (or intentionally).
... what, did you expect me to list all of that stuff here? I'm not going to spoil myself! Okay, well here are some of the subjects, minus the spoilers, that have been discovered. Guilds, clans, ladders and seasons have all been found referenced in the code. Some bosses are named and their in-game models have been discovered, along with several creatures. A boatload of Crusader skill details, including model animations are in there. Then on top of that details on game modes, including Adventure Mode and Loot Runs. That's just a taste. Head over to DiabloFans for so much more... if you dare.
Diablo III: Reaper of Souls is planned for release on PC in 2014, with no work on the expansion yet coming to consoles. First one, then the other, okay?