Something I've been wanting to do:
Have an important NPC be a girl in a frilly dress (making it hard for her to move quickly). This NPC is unarmed and if given a blaster will be aiming to miss (a horrible shot, noncombatant, what have you). The game will largely revolve around the decisions the humans make in regards to her.
The first night mission will involve retrieving the young girl. At this point, she is assumed to be the hiring scientist's young daughter, just a scared little girl who's utterly and completely terrified of the zombies. However, when she screams, zombies seem to gather around her. In game terms, whenever the NPC screams, all stunned zombies instantly become unstunned and may attack.
Assuming the girl dies, the game continues down a completely different road. However, in order to have her die, she must be tagged repeatedly or the humans must fail to rescue her in under the time limit given for the mission--a simple tag is not enough. She's not dying when the zombies get her.
Assuming the girl is rescued, she tries to escape at every turn, and if the humans talk to her, she reveals that she absolutely does not want to go back to the scientist, even pleading for her death rather than going back. With some more pressing, she will state that she is a test subject who has been tormented in hopes of finding a cure for the Z-virus.
And it worked, too. If the humans bring the little girl back to the scientist, he will vivisect her and create a number of cures from her remains (and the next few missions will become more difficult/bleak due to human cruelty). If she is mercy-killed, her corpse can still be used to create a single cure. But if the humans refuse to follow the script, rescue the NPC, but do not turn her over, she will follow them around for the rest of the game, causing major problems during missions (she'll still scream), but will ultimately be able to cure the disease, allowing for a kind of Best End situation if everything plays out correctly.
Basically, it's a potential throwaway escort mission that, if the right people make the right decisions, can wind up really changing the game. I guess I've spent too many years GMing, though: I really enjoy giving players a choice of how they'd like to screw themselves.