Few suggestions

[m2n2l2 0]

1.7.1.4 is crap, it really is. Zeds see you ~100 meters away and once agroed, summon other zeds to go after you - even if those zeds were behind the walls. Melee sucks and I've maintained amazing 14fps instead of the ~30 with the current patch. Anyways . .

Revert the game back to the way it was around 1.7.0 . . then give the game more of a roleplayaspect. For example, the engine has such thing as recruit mode, which allows you to see the enemies/friendlies/zeds marked with dots. Now . . give advanced players with certain amount of kills (say, 100) this skill with highly, highly limited range (say, 50-100 meters). Then with every 100 new kills, increase the range. Murderers should not gain access to the skill at all. (Perhaps even limit the skill so you only see bandits).

If you have 100 headshots, base weaponaccuracy could increase ~2-5% and with every 100 headshots . . well, you get the point. Point is not to give advanced players a big advantage, but a SENSATION they've achieved something and therefor gained access to something cool. This would make players also more protective and dedicated to their chars; now there is no penalty if you die near the beach and run fast to loot the body with the new char. I've done this and I'm sure many others have too.

I'd also consider making areas with "locust" swarms of zeds . . killing them all will summon a "master" zed which will have a low droprate of rare, rare weapons if killed. This would unite and lure people to do a "bossrun" rather than go shooting each others in hopes of good loot - if losing the skill to see your surroundings through the recruit skill isn't threat big enough.

Skills shouldn't really be that hard to implement. You just track certain features (like how long the char has travelled by foot and then give the player x% more walking/runningspeed).

Drug withdrawal. Do morphine, suffer from minor withdrawal symptoms (cold, shakes).

Safezone for friendly people to meet up and even trade.