How different would the game be? and what would you think Bungie could add that 343 didn't?
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Only two things I'm sure of is that the story wouldn't have been as good (unless Staten wrote it, maybe) and that the books would have been ignored. From a gameplay point of view, they would have kept a lot of the things that were left out, but they wouldn't have been as ambitious with it. Overall it would have reflected their declining interest in the series, like with Reach.