Everybody is speculating as to what Bungie is going to bring to Halo 3, rumors from new campaign levels where you play as a Marine (and not the Chief), to more multiplayer levels, or even both. Bungie, so far, has released maps in three waves so far: Heroic, Legendary, and Cold Storage. With the previous three maps releases there has never been this amount of hype nor something to the magnitude of a teaser trailer. So what I'm betting on is a full out Halo 3 Expansion, Halo 3: Mythic. You can either buy this on a disc or as a full download strait to your Xbox 360 hard drive.
This is a picture that was taken at PAX '08 where Bungie first revealed the new map Assembly. At the time no one knew what Mythic was, and Bungie just brushed it off as a "PAX version"
As you can see since the update to Halo 3 a few days ago, some interesting achievements have popped up in reference to the Mythic Maps.
Confused? Me too. At this point there seems to be multiplayer "Mythic Maps" coming at some point, but if you download the new Halo 3 video from your Xbox 360 on the Marketplace, the describtion does say, "New Campaign Experience". So it seems that we have multiplayer and singleplayer content coming. But just as with any Bungie products we won't know anything until Bungie wants us to know something.