Well first, based on the idea of zombies getting a buff, imo both zombies and survivors should get buffs as the game progresses. Survivors can 'create' items, while zombies can 'mutate.'
The creation has so much possibility, you could do an improv. system where players take random stuff they find and put it together into something usable.(car-bumper sword anyone?)
Think loot cards of respective classes. As in, say a player destroys a car in search of usable parts, they draw a card from the 'vehicle' card deck. Then, the rest of the parts are too destroyed/aged/whatever to be usable. You could even include a part upgrade system where players sharpen the metal(or whatever respective action there is) from that car bumper.
Then for the zombies mutation system:
As zombies are exposed to more and more of the disease(or whatever) that caused the initial sickness(from eating, be it survivor corpses or found dead animals or something else) they either mutate(system 1) or upgrade(system 2)
I don't mean for both system 1 and 2 to both be used, they're both separate ideas.
System 1: Zombies pick a card out of the mutation deck, they could end up loosing a body part, or get overpowered corrosion spit, or screech to buff nearby zombies with +move and +attack. This could totally alter how a zombie plays inorder to better the zombie faction as a whole.
System 2: Zombies accumulate 'upgrade points' and they can save points for a good upgrade(s), or spend the points on cheap upgrades. This would lead to early on decision making and tactic planning(I.E who does what) This would lead to a more, zombie rpg feel though.
Also I wanted to say, zombies could 'ladder' to basically teleport fellow zombies, unless I read it wrong. Assuming a zombie next to another zombie gets an additional point, then zombies could sort of do this:
X = Zombie to idle
0 = Empty square
Z = Zombie to move
X0
00X
X0
00X
X0
00X
X0
0Z
Z Currently has 2 move points, 1 from his initial one, and 1 because he is adjacent to a zombie. He moves up with his point and then move up again, creating a loop of free moves. The only thing is, this would require a lot of zombies to actually pull off.
Anyways, hope you like my ideas and sorry for such a long post.