My theory is that it will be an mmofps. Though an uncommon combination Bungie would be the ones to try and and make it work. I found evidence that it probably will be an mmo because they are hiring an "economy designer" which is only necessary for a large scale economy found in an mmo.
Also I think their ambition is that they want to create a cross platform mmo between 360 and PS3, regardless of how difficult that may be. That could explain the large number of PS3 engineers they are getting and the language of Bungie employees saying how ambitious they are for this project.