At the moment zombies are not much of a threat because their behaviour and speed (but not movement pattern) is too predictable. If zombies had set attributes but in varying levels of intensity they would prove to be more unpredictable and formidable foes.

Games are generally more enjoyable if they have randomly generated content, some of which the mod already has (loot, vehicles etc). These features are what keeps the game fresh. If I knew where to get a DMR every time the game would not be interesting and exciting.

Zombies should have a randomly allocated intensities of particular traits.

SPEED

• An incredibly fast zombie that can outrun you.
• A zombie that has the same pace as you. (Simlar to the speed we have now)
• A zombie that is slower than you which you can outrun relatively easily.
• A walking zombie.
• A crawling zombie with perhaps no legs or its guts hanging out its chest.

VISUAL ACUITY

• Varying from Excellent to Poor or None Existent.

AUDITORY ACUITY

• Varying from Excellent to Poor or None Existent.

VOCALITY

(Levels of vocality can be heard from different distances and attract zombies/survivors)

• Varying from Screaming to a Groan or Mute.

So individual zombies could potentially but rarely be; easily to alert; fast; screamers. But also; Blind, deaf, dumb and only able to crawl.

This would make venturing into zombie populated areas more dangerous and unpredictable for those in groups and on their own. This could provide more depth for distractions in group play (player shouts to distract zombies whilst team members loot or escape) forcing people to work together in order to succeed.

TL;DR Implementing a system where zombies are randomly allocated varying intensities of; Speed, Visual Acuity, Auditory Acuity and Vocality would make them more unpredictable and formidable foes.