i think they shouldn't necessarily make a sequal, but instead create a more involved game with only one character that focuses on survival-horror rather than survival-massacre, because after all it is a zombie game, but instead of frightening and horrific its just incredibly stressful, i'm not saying thats bad, by any means, its one of my favorite games
however they do everthing so well in L4D that i think if they tried they could create a zombie game to rival Silent Hill in creepiness, and the original L4D in pure awsomeness
EDIT: except for when the witch is crying and your not sure where she is, thats pretty creepy