Its excellent campaign notwithstanding, it was Halo 3: ODST’s Firefight mode that really ruled the gamewaves. Just don’t expect any DLC for that, ever.

“[DLC] was something that we didn’t really think we could do for ODST because traditionally all of our DLC has been about competitive multiplayer,” says Bungie producer Curtis Creamer. “That’s something that we had the systems in place for. But, since Firefight is more based on the cooperative campaign mode, that was a technology that we hadn’t built up to a point where we could add DLC to the game. It would have been too much of an investment to make that happen.” 

Or the developer is just too busy working on Halo: Reach to bother.