Microsoft has always been very interested in expanding the Halo universe beyond its core line of shooters. With books, comics, and a potential film project with Peter Jackson, Halo's appeal has always stretched beyond the games.
There was however a point when Microsoft was just as interested in building new types of Halo game experiences to broaden the fiction. Project Titan, better known as The Halo MMO was to be the culmination of these efforts, offering players a fully explorable world full of Halo lore and stories.
Unfortunately, after the first attempt to spread Halo beyond the FPS flopped with Halo Wars, Microsoft shut down developer Ensemble Studios and in the process canned Project Titan. Alas, maybe the world wasn't ready for a subscription-based game full of 12 year old teabaggers.