From Blizzard's New MMO Wiki
The project Titan came up on a supposed leak of Blizzard's product-slate  with a launch date of Q4 2013.
Titan is... the media is not meant to know anything about that. It's our next gen MMO and we've only started talking about it in a limited fashion because we want to leverage the fact that we're working on something like that for the purpose of recruiting – getting some of the best talent in the industry on that.
The title Titan is almost certain that it is a code-name, so don't expect an MMO based on Greek mythology or Titans.
Blizzard has a record for using this style of code-names for its projects:
- Starcraft II had the code name Medusa
- Diablo III was named Hydra
- and now the Next-Gen MMO named Titan
Ensemble's Cancelled Titan MMO
Titan was also the name of Ensemble's cancelled Sci-Fi MMO. There is an article about this coincidence, concluding that it's just a strategic move from Blizzard gearing up for a Sci-Fi MMOFPS but not going for a Halo MMO.