One skill tree: Survival. Invisible experience system. No one needs to see how much xp they have or need until the next level. Experience is awarded for active playtime (acquiring items, using items when necessary hungry/thirsty/injured, being in the proximity of other players, etc). None of the skills should significantly alter the way the game is played, just add minor conveniences that make survival a bit simpler. With each level basic stats improve minutely. Slightly greater resistance to hunger and illness, slightly improved blood gain from cooked/heated food, maybe even very slowly increasing blood regeneration. With certain levels you gain basic skills like starting a fire with a tinder/firewood item you loot off the forest floor instead of matches. Getting more meat from gutted animals. Being able to heat canned food for a little more blood. Every five levels, choose to make movement either faster or more quiet in all postures. Doing it like that would leave the core of the game the same while giving the players even more incentive to want to stay alive. You'll feel worse when your guy dies, especially if you've stayed alive for a while. Plus, stuff like improved mobility might tempt players into taking more chances the longer they've been playing. A good guy might think that he's quieter or faster than the bandits he's encountered and start hunting them lone-wolf. A bandit might get ballsy enough to think he's quiet enough to sneak into a camp and loot tents. No matter what, it should be small, simple, and sit nicely on top of the foundation at the core of the game. Reward players for surviving and cooperating. I think what makes this mod so special is the permadeath aspect and how it really changes the game experience for most people. Adding skills that allows you to see your character grow as you play could make that even more rewarding when you survive and devastating when you don't.