From Diablo Wiki
Unannounced titles remain so until a developer feel it's time to show their work to the world. There are advantages and drawbacks with keeping a game under wraps.
Blizzard's Unannounced Projects
Blizzard currently has two unannounced projects.
Project Titan is the internal codename for a Next-Gen MMORPG, under development since 2007 and very tentatively targeted for release in late 2013. Titan is a new Intellectual Property (IP), and not a sequel to another Blizzard game or a game set in any of their existing game worlds. The project is well under way and Blizzard has been opening posting jobs on it since 2009. No public information exists about it, and only the codename is known at this point.
Though the project has been underway for several years, and has an internal target date for late 2013, Rob Pardo stated[] at Blizzcon 2010 that there was no chance it would be revealed until at least 2012.
Blizzard's other future project is not as far along as Titan and virtually nothing is known about it, other than that it exists and that it is set in another new IP. No public hiring notices have yet been posted for it, and it's safe to assume that it's at least a couple of years behind Titan in development.
Why Not Announce?
Blizzard has gained a reputation for announcing their games far in advance of their release dates. It's good to start the publicity and anticipation building for a title well in advance, but too far ahead can leave fans dispirited or burned out. (Though secrets can be hard to keep, as the project grows larger and many dozens of people are working full time on it.)
This is definitely an issue with Diablo III, which was announced in June 2008, and yet will not be released before late 2011 at the soonest; three and a half years after the reveal.
Blizzard employees have repeatedly said they take too long to make games and that they need to hold their announcements until the games are further along in their development cycle. This is difficult since they do not know how long the games will take to create, but it seems safe to assume Blizzard will try to keep the secret of their upcoming game titles for longer than they managed with D3.
Ongoing projects are given code names that may or may not have anything to do with the games themselves. After all, the developers and others in the know at the company have to refer to the game by something that won't give away the true nature of the endeavor.
Diablo III was referred to as "Project Hydra" during the 2005-2008 development at Blizzard North, before the game was announced in June 2008.
|Blizzard Entertainment & other Game-related Topics  Blizz Games Other Games Terms Devs Blizz ppl D3 D2 D1 Other ppl Events Merchandise Related|
Blizzard All cont.
|Diablo III Team|
|Diablo II Team|