Varying Zombie Traits
Basically where different traits are either activated by a % of spawn rate or "types" of zombies as in "freshly turned, regular, and old/decayed".
---Also note, that this could be controlled by options or difficulties/other mechanics. Doesn't have to be completely game-altering.
Some could sprint while others quickly shamble, shamble normal, or shamble slow (or simply fall over like they're mentally handicapped and crawl or struggle to get back up )
Some could also have varying levels of hearing/sight/smell/strength/health depending on either chance or 'realistic' circumstances. Such as the state of decay they've reached or even THEIR inventory. That is, the professional zombies in the new update might be bogged down with gear and move slower, or the ones from the universities might have a bag with books in it... just a thought (watch out for the fresh/naked ones I guess ) New Skill - Parkour
Not anything super crazy, but maybe the ability to safely land a roll out of a second floor (if skill is leveled, or if perk gained from profession).
Able to jump tall fences unless barbed or whatever.
Able to climb in and out of windows, over counters, over small fences quicker (and maybe retain some momentum)
Able to climb up walls for 1 story, flat-roofed buildings and get on to the roof.
Could be balanced with % chance to trip, injure yourself, quicker stamina drain, etc. depending on skill level or even just always.
Rain could also effect this (higher chance to slip and fall - could be fatal/debilitating if the drop is large enough) NPC Behavior
This one is a random thought I had -- have some hostile NPCs attack you intelligently. Such as once they know where you're staying, to stalk you a bit and when you're inside they may throw a noise maker in there to flood you out and potentially be somewhere waiting for you.
Random ideas, lemme know what you guys think