•Fatigue system, the higher your fatigue, the lower your chance of evasion, critical hit, slower attack speed, slower movement speed and a chance to miss (this takes a long time to set in), fixed by some consumables or by heading back to a sanctuary
If you are not in combat or near enemies it should also regenerate. Such mechanics usually become a headache otherwise and annoy far too much. The rate can be very slow and need several minutes, but it would allow for some deeper game play where you need to manage fatigue carefully.
•Map progressively gets harder, but the higher the danger the greater the reward
If you mean with respect to time, then you need a way for high end players to skip the easy parts. No one wants to wait 2 hours just to end up continuing where they were playing at last session.
•Ammo and guns are very scarce, and such very precious
Avoid cumbersome ammo systems in a RPG. An example being having dozens of weapons and each needing special ammo unique to them. Not only do you end up annoying players who find all ammo but the type they need, but you also end up having items everywhere that no one can use.
If you are using WC3 inventory for other items and weapons then consider merging ammo into the weapon itself. Inventory space is too limited otherwise.
•3 attributes will be: Weapons, Protection, Survival (possibly, not 100% sure)
You could add luck to it. I mean its a zombie survival so why not? Standard boosts evasion and critical chance very slightly but also has other less immediately noticeable effects. Searching houses yields better drops (less chance of being empty, more ammo if ammo is found etc). Also when it comes to zombies it alters things slightly in the players favour. Higher luck teams could generally get fewer zombies with better returns than low luck teams.
Many players like controlling more than 1 unit in a game. Since this is survival based, why not allow control over lesser survivors? These would need to be found and be quite weak but act as an extra weapon and help just a little bit. Their strength would obviously scale with your attributes, possible even giving them a fraction of your attributes. If it features classes, an entire class could be dedicated to this sort of play, otherwise you could add specialist items required to find and rescue survivors (allowing a hero to branch into the role).
Difficulty levels are pretty standard in RPGs. Try to make it so that at higher difficulties more stuff happens other than monsters gaining stats or more/better spawning. For example, at really high difficulties you could throw out 2 mini-bosses at once, or throw them out more often. Although weapons improve, they become harder to find and less ammo drops. New types of monster spawn that penalize soloing etc.
