As an almost complete departure from my normal content, I wanted to share a theory I had about Halo 3. This post contains some very mild spoilers, so if you're still not a good way through the campaign and trying to avoid EVERY spoiler, you may want to wait before reading it.
Spoiler Dead Space
Anyone who's played most or all of the campaign solo knows that you only play as Master Chief, and the Arbiter is around as an NPC some of the time (co-op is different, but I believe that that's placement of convenience rather than story cannon). In fact, the "dropping" of the Arbiter story line has been complained about in a number of reviews. This leads me to the theory:
There will be a Halo 3.5. Or 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. It might even be able to be sold as downloadable content off the marketplace. And it would make even more money off negligible changes to the Halo 3 engine. And then they can re-sell "Halo 3: Game of the Year Edition" with the Halo 3 and Halo 3.5 content. Yes, it's a very Bethesda-esque route (much like Oblivion has done), and it's one Bungie avoided in the past (given the theory that Halo 1.5 would be released with Xbox Live multiplayer). But it could happen, and that's just my theory.