Ensemble Studios (at that time part of Microsoft) was working on an MMO from 2004 to 2007 when it was cancelled. The interesting fact is that Ensemble's MMO had the code-name Titan as well. So, we just had a hint of Blizzard working on an MMO with the code-name Titan, when several years ago another studio was working on a similar project with the same name.
Check screenshots from early versions of Ensemble's Titan MMO.
I will admit that you see what you want to see but I will go this road anyway. So what links these two projects?
Like mentioned above, both MMOs have the same code-name, but we should note one thing. During the development of Ensemble's MMO, there were some leaks of the project. The studio then decided to change the code name to Orion in order to deal with the situation. That does not prove much since the developers that were working on it, still refer to it as Titan instead of Orion.
Science Fiction theme
Ensemble's MMO started off as a "simple" SciFi MMO and later it was decided to use Halo's universe. BlizzMMO is also to be Sci-Fi themed. So both MMOs have the same genre.
Ensemble staff joined Blizzard
On 2009, Ensemble Studios was officially disbanded, at that time several people from Ensemble were hired by Blizzard. If you play World of Warcraft, you probably know Ghostcrawler (aka Greg Street), currently lead systems designer of WoW and former lead designer of Ensemble's Titan MMO as well.
Also, some artists from Ensemble are now working for Blizzard, as well as one of the lead programmers which is now the lead graphics programmer.
While from Blizzard's point of view, a respected game studio was working on a Sci-Fi MMO and disbanded, making available experienced professionals that have explored and faced the challenges of a Sci-Fi MMOFPS who could contribute their knowledge and save precious development time.
Halo & Bungie
Bungie is known for creating the Halo universe. Couple months ago a 10-year partnership between Activsion-Blizzard and Bungie was announced. I remind you that Ensemble's MMO was using Halo's universe. Interesting, eh?
There are several hints towards the theory that a Halo MMO is being developed either by Blizzard or Bungie. But the reality is that at least by those two studios it's not happening. Why, you ask? Because it is already verified by Blizzard officials that their new MMO is a brand new IP and is made entirely from scratch.
We have nothing to advertise for the new MMO because it's a shell of a game thus far. We've already stated it'll be a brand new franchise, which means the lore, art, and game play are being developed entirely from scratch. It's an overwhelming process, but a process through which we excel. Our track record supports this.
What about Bungie? Couldn't Bungie make Halo MMO?
Most probably, no. On the press release of the 10-year partnership with Activision, it was mentioned that the deal was about "Bungie's next big action game universe". Also, Halo's IP belongs to Microsoft, so it is highly unlikely Microsoft will agree on licensing Halo and lose a big bunch of the pie.
So, it is obvious to me that while ideas from Ensemble's dead MMO may be used on BlizzMMO, it is almost certain that either of those studios will develop the Halo MMO.