Charlize Theron. A bit cliche, maybe, but she would work really well.
I've actually made a super basic outline for a live-action movie years ago. I'll put it up later.
(And no, you couldn't have a movie like the games, it doesn't work. You'd need non-shooty downtime, and dialogue, and personal interaction and other stuff)
It's later! Edit:
Act 1: Samus has been hired by the Federation to consult on a recon-in-force/raid on a Space Pirate-controlled planet. She will ride along on a dropship with 1 detachment of marines, and is not allowed to bring her own gunship (she's not happy about this, but it's a paying job).
The other two dropships are shot down immediately, and the primary one is hit and crashes. Many marines are killed or badly wounded, only about 10 marines and Samus survive.
Act 2: The marines need to find a sanctuary. So they all carry the wounded away, detonate the dropship, and hole-up in an abandoned building or something. The Squad Captain and Samus need to find a way to contact the Federation to call for rescue. They find a relay station, and learn that they can't be rescued yet, because of the defense grid. At least not without a costly full-scale assault. Samus and the Captain find the location of the Pirate's defense center, and decide that their only chance is to sabotage the defense grid to allow a rescue shuttle to land. During periods of rest, Samus and the Captain have conversations, and begin to develop a relationship. Tension!
Bonus Act 2 Subplot! The rest of the marine squad left at the hideout engage in a tense game of cat-and-injured-mouse with a patrol squad of pirates. They must prepare a defense against an attack, while trying to stay as hidden as possible.
Act 3: Samus and Captain infiltrate the Space Pirate defense center, managing to stay hidden most of the way in. As they approach the center, they are finally spotted, and need to fight the rest of the way in. They encounter lots of increasingly well-equipped pirates, defense turrets, and aggressive pirate-trained creatures.
Meanwhile, the marine squad begins the battle with the pirate patrol, guerrilla style. IEDs, snipers, electronic warfare, and even some melee combat; think Saving Private Ryan or Black Hawk Down style.
Samus and Captain arrive at the command room, locking the door behind them; they're battered and bruised but alive. There, they meet the Pirate Base Commander. He's big. He has a pet: A Metroid! *shock!*
Cut back to the marine squad: They've taken casualties, but won the engagement. They're taking care of their dead and wounded, and taking a brief rest, when they realize that there are pirate reiforcements coming. The able marines go back to prepare for the next probably dooooomed battle.
Meawhile, there's a fierce battle in the command room. The marine captain is mortally wounded, and Samus gets steely and angry. She kills the Metroid and the Commander, and holds the Marine Captain in her arms as he dies. It's very sad. Then she sabotages the defense grid, and uses the comm system to call for rescue... and something else.
Cut to a Federation dropship flanked by two Federation gunships, streaking down toward the city/base/whatever it is. Pan up, Samus' gunship is heading straight for the command center. The Fed gunships engage Space Pirate aircraft, while the dropship moves to rescue the marine squad. There's a battle as they move their wounded into the ship under heavy fire. Rockets are hitting the side of the dropship and alarms are blaring, and then Samus in her gunship rush in and strafe the living **** out of the pirates on the ground. Then they all flee back to orbit.
Samus is debriefed by the Federation Admiral or something. It's revealed that she downloaded a bunch of data from the Pirate mainframe, and she tells them about the pet Metroid. The Admiral looks shocked for a moment, then you can see he's made a decision. Next scene is Samus walking out through the door, she grins as she puts her helmet back on. Roll Credits.
