Everyone seriously interested in what a possible Halo film would look like should read Vociferous's (formerly from Ascendant Justice, now with 343I) own script.
It follows the main story of Halo:CE with some added scenes and side plots as well. It follows the Chief mainly, but also follows Thel 'Vadumee (the Arbiter in Halo 2) as well in order to set up the potential Halo 2 Movie. Ingenious script.
Again, I can't take credit for this. All credit goes to Ascendant Justice and Vociferous.
http://ascendantjustice.com/files/voc/film/halo.pdf