Diablo 3 - Getting an Expansion in 2013

Mike Morhaine from Blizzard Entertainment confirmed that an expansion for Diablo 3 will be released in 2013.   It happened during a 2012 earnings call. Gamespy speculates:

Could the expansion alleviate many of the issues gamers had with Diablo 3? That's far too hard to tell at this point, but it is worth having a fun bit of speculation regarding what exactly will be in this particular expansion. I figure the story will revolve around Leah's mother, Adria, and we'll probably get one or two more playable characters. What do you think we'll see in this expansion and/or what would you like to see happen with Diablo 3?