This movie has to be R, I don't care if they have teletubbies in it, as long as the rating is R it will do fine.
The cast for the marine was really good I think, Karl has the looks to pull this off and it's not really that much of a challenge for any actor to just shoot a weapon for two hours.
As far as storywise I think this movie has to start without giving the viewers any idea of what's going on, but have the people he meet and stuff he sees at camera, hear over the radio and stuff like that tell the story. Like in the game, but more in a way so it will suit the movie.
Making this movie will be a minefield, they can't have bullettime in it, they can't have techno music, slow motion scenes. They can't have gangsta black guy doing the funny one liners, they can't have a sexy lady with big boobs playing the "I hate you marine - oh it's the ending lets make out".
The movie obviously has to have tons of CG, but it would be really cool having most of the monsters "real" as in puppets or whatever, it gives the movie a better feel to it, with CG you "know" it's not real so it kind of ruins the immersion if it's just too much, which in a movie like this can get really annoying.
If Rock gets a role in this movie I hope he is a hardcore soldier/Sarge with no "prettyboy with muscles" look.