Honestly, I think they’d be fine to mirror the game series. The first Halo game described who/what Master Chief and Cortana are very well, it described what the Halo worlds are and why they are there, established the Covenant and the Flood. I haven’t played “Halo 2”, but “Halo 3” was also well-told and a good cap to the saga. Then follow up with “Halo: Reach” to show how the events unfolded into what happened in the Halo Trilogy.
Of course, the question must be asked—does a movie series even need to be made? The games did such a great job of telling the story, I can’t help but think movies just open up the possibility of ruining the story. I think the same of “God of War” for that matter.
One good reason for making one or a few Halo movies is that you could actually sit back and watch it unfold, without worrying about dying, and enjoy it. I know there’s the whole issue of immersion in gaming, but if a Halo movie is made well enough, it could chart on the record books.