Update: Videogaming247 pointed out a gallery full of concept images from the Halo MMO project via Flickr.
Ensemble Studios, developers of the upcoming 360 RTS Halo Wars, has recently canceled an MMO project involving Master Chief's universe, according to Gamasutra's report. The studio's decision to pull the Halo MMO was due to the fact that Microsoft decided to shut down Ensemble after Halo Wars was completed.
Developers of the hit RTS series, Age of Empire and Age of Mythologies, the Halo MMO was to be Ensemble's first project under the genre. Reports of the title came in since 2006 when publications, including Game Informer magazine, said that Bungie and Ensemble were teaming up for a project. Also, months prior to Microsoft's decision of shutting down the studio, Bruce Shelly from Ensemble Studios stated that the studio "set up three prototype teams out of the staff of a major project that we cancelled," on a blog post in June. Shelley also revealed in the post that two prototypes were cancelled after six months of work, "with one getting more time to demonstrate the value of its concept".
According to Gamasutra's report, Massively.com lead blogger Michael Zenke said that the Halo MMO was set to introduce new creatures never-before-seen in any Halo title. Referring to the screenshot obtained by Gamasutra above:
"The character pane shows a health bar and 'mana' -- or Psion, as is referred to in this title... The upper right icons are for basic character functions. The one on the far left would most likely be inventory, represented by the outline of a man."
Microsoft and Bungie did confirm that new Halo titles are currently in development, but none of the projects nor rumors generating from both companies indicated any titles under the MMO genre.