How would you make it? What section of the Haloverse would you focus on?
Personally, I'd try to remove it from the Action category.
Yes, it's practically a war movie, BUT I believe the movie could be so much more than just a mainstream hit movie.
An adaptation of Combat Evolved, with a good deal of elements from the novel: The Flood, i'd start it being an action sort of movie, with the Covenant being almost SCARY, bring out the fear aspect of it, make the Covenant not just another enemy, make the Marines and Crewmen genuinely afraid of Elites, and very cautious of even grunts. Show the true nature of them being completely alien.
Keep an almost thriller sort of feel to it for the most part at the start, the Chief HAS to get off the Pillar of Autumn with Cortana, leaving Keyes and the rest behind, but he has to fight through hordes of baddies just to get out and he's running out of time.
When they get to the surface of Halo, it swaps perspective to the Marines and ODST's, and focuses on them, but they refer to Keyes and the command structure a bit as well, showing them being captured and brought to the Truth and Reconciliation.
The story pretty much continues on being a sci-fi Action movie, until... the level 343 Guilty Spark.
This is where the focus really starts to shift to the Chief. Follow the marines closely through the 'Weapons Cache'... make things creepy... start to get the real feeling that there's something even worse than the Covenant, which was already a totally feared enemy... and turn the movie into an action/thriller/horror. Things get ultimately scary, a Virulent Parasitic Alien Lifeform that reanimates your friends. Give the audience nightmares with this stuff.
Bring it along like this, watching the Covenant run from the Flood, the Chief fighting through masses of the undead, up until the return to the Pillar of Autumn. Let the creep wear down slightly, return the action to it, and have the run to the Longsword be the most epic you could possibly make it, with a calm, almost serene ending of relief of survival.
Go ahead, discuss what you'd do different. I often think of what the other true fans of the Halo Saga would want in an alternative form of the Haloverse.