I find the current focus of development to make the surface world as hard as possible to be somewhat misguided. You see, it does not matter how strong you make the zombies or how hard to kill they are when they are standing in the middle of a street right in the open. You can fire at them from as far as your ranged weaponry will go, lure them into traps or trample them under a vehicle and short of nerfing these methods to unrealistic levels (cars flimsier than paper, guns shooting airsoft pellets at one foot of distance and spiked pits just being minor inconveniences) Nothing done will rid an experienced player from feeling that the game is “easy”.
At best, it will just make newer players feel like the game is impossible… Until they figure out the most reliable ways to deal with enemies in the open, then, they will join the players talking about it being too easy.
What the game really needs is, in my opinion, some focus back on dungeons. These more naturally restrict the player’s options without recurring to strange, unnatural roadblocks (“Your gun can only fire 10 feet in front of you in broad daylight. That way, you won’t find it easy to snipe zombies from afar!”) by providing an environment where the conditions are naturally adverse. A ranged weapon can only shoot as far as your light source can reach, a vehicle can’t squeeze inside an underground temple, enemies can dynamically spawn in unvisited rooms without the player wondering about it, solar power is meaningless when you are ten stories underground and you can only carry so many resources on your person while exploring.
Besides, underground areas are overdue for renovations.