Halo 2 Expansion?
Gamespot has coverage of the appearance on an official Korean Xbox site of a "Halo 2 X-Pack". The possible expansion is another possible way for Bungie and Microsoft to get the heavily foreshadowed ending to fans of the series. From the article: "Such a Halo 2 expansion could be a twofold boon for Microsoft, adding another incentive for gamers to join its online service and squeezing further dollars from the nearly 7 million gamers who already have Halo 2."
I believe this is just new content and patches for online / multiplayer of Halo 2. New maps, skins, bug fixes and the like.
Halo3 will most likely be released with the Xbox2 or just afterwards
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