By the power of truth, I, while living, have conquered the universe.
I don't really think a second ODST game could work on the same level that ODST itself did. The narrative style, atmosphere, tone, just about everything about it was completely unique to Halo because it was such an isolated story following the conventions of the film-noir genre and being structured much like Dante's Divine Comedy with all the allegorical references.
I'd like to see a DLC expansion (yes, an expansion for an expansion) simply adding to the current ODST experience - an Elite side-story where you play as one before the Brutes betray you, and a mission or two from Dare's POV showing her escape underground. Some new environments too. If that were done, the game would be nothing short of perfection.