Right now, I think the wilderness is too safe. I know I suggested it before, but I couldn't find the old topic and I know the devs were recently asking for suggestions. One of the ways I think you could make wilderness survival much more difficult is to allow zombies to hide and cross through water.
Survival would be much more difficult if zombies were not walled off in some areas (like the lake near the cabin) by large bodies of water. If they were able to, albeit very slowly, travel underwater, and pop up with little warning, it would add a whole new dimension to surviving in the wilderness. No longer would it be safe to fish all day by the lake, knowing that the zombies can't cross or hide in the water. You would have to take extraordinary precautions to circumvent this mechanic if you wanted to avoid it, making it a good addition I think. Players would not only have to worry about zombies who may already be under the lake, potentially hearing them fishing, but they would also have to worry about being spotted across the lake by zombies. If their not attentive, zombies could jump out of the water at a very inconvenient time
While discussing water mechanics, I think the players should have the ability to swim, although it could be a costly perk to take. Non-swimmers should be allowed to enter water as well, but if they go too deep, there should be a very high risk for drowning (multiplied if there is any zombies directly underneath the player).