I thought it would be cool to introduce progressive skill building and skill trees to the game. Skills that improve as you use them.
It would be cool to have different areas to build into as well… ie. mechanics (cars in rust?!), traps, weaponry, hunting, tracking, foraging, cartography, woodworking, metal working, architecture, etc.
It would add another element to the game and give people something else to work towards in later game rather than just running around killing nakeds and raids.
For cave exploration, it would be interesting to have a few caves throughout the game with harder/more dangerous animals in it (bear/wolf dens) with a higher population of resource spawns/ barrels. Some could also have entrances located under water that you have to swim to, on high mountain peaks only accessible by building a tower or bridge, hidden in a dense forest, etc.