Follows Contact Harvest to the letter. The book is perfect for a movie adaptation, and it would be a perfect place to begin the story of the Human-Covenant War. The movie would end with refugees escaping Harvest, and the Tiara plummeting to the surface. Then it would end with the Covenant glassing Harvest. It would serve as a perfect explanation as to why the war started, and would be a great 3-hour movie.
To counter arguments against it, the Harvest story is hugely necessary. As I stated before, Contact Harvest was an amazing book. Viewers need to know why the initial negotiations between the Humans and the Covenant went bad, why the Prophets wanted to eradicate Human-kind, and they need to see the full civilian impact of what the Covenant were doing to the people of Harvest, and later colonies such as Madrigal. Plus, it gives Johnson his moment in the spotlight, and will shed light on the UNSC's situation (the dirty war with the Innies) before the Covenant, which take place in the first few chapters in the book.
Starting with Spartan indoctrination and early parts of the war, this movie could span through Fall of Reach going through everything that happened, including Sam's death and the Battle of Reach. Half way through, it could then transition to Installation 04, including relevant material from Halo CE and The Flood, finishing with the Halo's destruction. It would be a great 4-hour movie; or even a 3-hour movie.
This film would be a mix of First Strike, Ghosts of Onyx, and Halo 2. If they did it right, the movie would follow the storylines, mix the scenes accordingly, and fabricate an amazing 3 hour movie packed with details and epic battle scenes, all tied-in accordingly.
The most obvious material to use for the fourth and final Halo movie would be Halo 3, and Halo 3: ODST. Perhaps they could add some stuff, maybe remove some stuff; use artistic license. Done right, this would be a fantastic end to the Human-Covenant War. This movie could be 3 hours long. Ending with The Ark's destruction and the memorial back on Earth, that would be the perfect Halo saga of movies, to maybe even compete with Star Wars. Plus, 4 movies would break the endless facade of trilogies plaguing the book and movie industry.