Bungie initially conceived ODST as a small side project to produce in the lull between Halo 3‘s completion and Halo:Reach. Instead of featuring recognizable characters such as the Master Chief from previous games, the developers focused on the ODSTs. Story director Joseph Staten penned a detective story utilizing film noir settings, design, and characters. Composer Martin O’Donnell abandoned his previous Halo themes to create a quieter, jazz-influenced sound. During development, the game grew in scope to that of a full-sized game. [info from Wikipedia]
When Bungie decided to make a new Halo game after 3, they tossed around alot of ideas and concepts, such as another game where you play as Master chief, or one focusing on the Arbiter. Eventually they came across the idea for a game where you play as an ODST, they went through several concepts of what it would be about, some early concepts for the ODST idea were a game where you’d play as a ODST squad that would follow alongside a Spartan. Another one was about going back to Waypoint. They reused alot of cut ideas from Halo 1,2, and 3 in ODST such as these cut Pelican models cut from Halo 3: Read more