I have a few. An FPS where you fight in one of the last settlements of mankind, besieged by zombies. Only head shots kill, supplies are scarce, and once the fort is overrun you have to make it to the coast, oh and while you can dodge attacks you only have one hitpoint. (Also, you need to eat food and sleep while you are playing other wise you either starve or go kind of insane.)
The other is a survival horror game, like Dead Space but good. You are landing on a star ship to visit your family the ship looks decent (Bright corridors, fountains, strategic plant life)
But the way the people are acting seems a bit off. As you are on the elevator you are attacked by a deranged mechanic, you blast off his arm but he regenerates it, and it is stronger with a different design. (Like a claw or hammer) You run like hell, and finally get to the central plaza where your family waits. A voice taunts you as your wife and children attack you. No matter how badly you destroy them they just regrow the damaged body part and make it stronger. (Occasionally include a different design so as opposed to a hand you get a sickle.) You escape, but the rest of the ship has gone insane, everyone is trying to kill you, the ship is getting torn the hell up, and the more you fight the stronger they become. You send for help and try to uncover the route of the problem and survive until transport arrives. (Left out the plot.)
Finally the third is a good old fashioned alien killing FPS (No space marines though.) An alien ship crashes on Earth, the bug like monsters attack reproduce quickly and in a few weeks have overrun the Americas... Okay it is the Zerg versus humanity essentially, but with a slightly deeper plotline, and a lovely set up for a sequel.