Make the player survive winter, and other ideas.
Lemme start by saying: I love you for making this game. You're brilliant and deserve to make lots of money.
Anyway, not sure if the winter thing would work within the structure of the game (would creating a season dynamic be possible?); if not, create a quest that triggers winter, and the player has a count-down warning during which they'll need to stock-pile food. A text screen could pop up to give player a cue to know fall is over (something like, "the skies turn a thick ashy gray--it looks like snow.") Then add 'lightly snowing' and 'blizzard' as weather conditions.
1. Allow the player to find seeds and set up farmland (you'd have to defend the food source during autumn when vegetables are almost ready for harvest). You'd also have to find jars and empty containers to store food for winter.
2. Allow the player to find a variety of safes and lock-boxes to safely store goods that don't fit in campsite.
3. Crafting Idea: rain-catching device.
4. Someone on here mentioned the idea of a water-carrying camel--love that idea. Here's another thing to consider with that: allow the player to fit the camel with a strap for the shoulder opposite rifle/crowbar.
5. Gear ideas: Socks? Thermal underwear? Snowshoes? Skis? (Those factor into the winter survival thing.) The way the game is now, I find that if I have a hoodie and a few shirts, I never have to worry about temperature. To make gameplay more challenging, there could be severe temp drops later in the game that force well-equipped players to pay careful attention to their body temp.
6. Took this idea from Darkwood: should be able to board up shelter and fortify for defense bonuses.
7. Agree with others: a level-up system with point distribution would make this game even more like crack-cocaine.
8. Love the direction you're taking with encounters. Would be cool to have other people in your party (wouldn't be over-powered, perhaps, because you'd have to feed them).
9. Quest idea: escort helpless person somewhere--could make for an interesting dynamic/challenge.