The third set completing the Return to Ravnica block has been finally announced just recently. The set is called Dragon’s Maze and to be released to the market on May 3, 2013. It has a total cards of 156, and it will include all ten of the Ravnica guilds.
Prerelease Events will be on April 27-28, 2013, and the set’s Gameday will be on May 25-26, 2013. More info the set can be found here.