I'd love the option to choose an optional goal that makes the game harder when spawning a new character. For instance a young child NPC who follows you around. They would require food, water and be more susceptible to the cold. They could also create more noise, scream when you shoot etc. If possible over time they could become stronger, faster and possibly be able to use a sidearm (but be very inaccurate and shoot at annoying times). Anything like this that adds a bit of extra challenge to the game. Alternatively maybe you could spawn in with a character who is overweight so they run more slowly. Basically any optional handicap to ramp up the level of difficulty for an experienced player without negatively affecting everyone else.