Gaming Today's Shawn reports a speculation that he could actually see happening:
Mark Methenitis puts forth an interesting theory in his latest "Law of the Game" entry. He gives an intriguing explanation of why Bungie dropped the Arbiter story line and players weren't able to play the Arbiter. This has been a complaint in several reviews. Methenitis believes or at least hopes that Bungie will release a Halo 3.5, Halo 3-2 or Halo 3 Directors cut:
"The storyline will basically be the Arbiter's cannon story during the activities of Halo 3, filling in all the gaps of absence in the Chief's escapade. And after all, why not? It would only be addition to the story, which would take a comparatively small development time."