I had this idea a long time ago, but it seemed impossible. However, now that I've seen people make mannequins with additional poses and even sentries that look and sound like people, I think it may be doable. (Though it would definitely take far more skill than I have!)

So here it is: An NPC you can place anywhere and interact with as you wish. Maybe some are vendors of varying types. Others could just have some dialogue options to interact with. You could even get them as a reward for a quest - after being tasked with saving someone from a zombie-infested spot on the map, they come with you (in your inventory... maybe?) and you can place them in your base to liven it up. This could really add a new flavor to single player games.

I suppose an easy alternative to this would be a re-skin of mannequins to add flesh tones and a face/hair. What does everyone else think?