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."
Leaving off an end the way they did only enforces this belief. Would Da Vinci forget about finishing off the Mona Lisa? No, he'd get it done. Now we have an expansion on the way....pump pump pump.
I have to disagree with you on your assesment of halo 1 levels. Remember the last few levels, especially the library? The library has to be the worst level design in history. Then the one right after it was just running through an earlier level in reverse. Its obvious that the game was rushed out the door and level design took a back seat to other matters.