I really should think before I post.
I'm always revising what my ideal story arc for "Doom" would be as a film, and instead of giving you a long, exhaustive treatment, I thought I'd sum it up in a few short paragraphs. I've discussed this before, but I wanted to be more concise this time. Since we all know certain background elements (such as the fact that a massive corporate entity controls all space travel and has purchased the military), I'll skip right to the juicy bits.
Marine on Earth assaults superior officer for order his men to fire upon civilians. Punishment is administered in the form of six months manual labor and menial duties on Martian soil. Military personnel and various technicians, architects, biologists and engineers make the long trip to Mars. Marine becomes bored as life is reduced to maintenance, labor, statistical reporting. His main release comes from reports from home and occasionally testing experimental weaponry.
UAC's attempts to create multiple one-way teleportation systems (or "gates") between Mars and its two moons are a success for both cargo and humans. Progress was sped up greatly by the discovery of an alien artifact found on Mars, which is actually a beacon. Upon being examined, it sends a signal to an unseen force, who has been waiting for humanity to discover it in order to ensure they are worthy opponents. A massive organic entity which resembles the Icon of Sin suddenly appears near Mars, as if materializing out of nothing. It is larger than Earth.
After swallowing Phobos whole as an initial threat, the Icon of Sin begins deploying various monstrous creatures, which quickly infest Deimos and Mars. UAC quickly sends two platoons of its elite to eradicate the problem. One platoon is sent directly into the Icon of Sin's mouth under the guise of saving Phobos, when it actuality it wishes to form an allegiance, attracted by its immense power. The second platoon makes its best attempts to clean up Mars before moving on to Deimos.
ACT 4: By the time the second platoon has made it to Deimos and is ready to "jump the gates" to Phobos for a search-and-rescue, the ulterior motive is revealed. In order to destroy the Marine and anyone who might get in the way of the proposed allegiance, a captured monster is released into Deimos, destroying all but the Marine, who manages to obliterate the threat. Upon traveling back to Mars, he and other survivors manage to flag a ship back to Earth, but it is too late. The first platoon's request for allegiance is denied and they instead spend the rest of their existence being tortured inside the Icon of Sin, who is now heading toward Earth as well.
So it's a bitter ending, not necessarily leaving itself open for a sequel but definitely not indicative of a positive resolve. Acts 3 and 4 would entail all the action, gore and disturbing bits. It would be an enormous payoff for a somewhat slow but intriguing beginning.