Mutations.

[Sgt. Lag]

Wouldn't it be cool if downed zeds randonly mutated from time to time?

For example you could decapitate a gorefast and a tentacle thing could grow in that place so they can still attack you

 

[XyzKiller]

Reminds me of Halo Flood.

But yeah, I guess something around those lines would make it a bit more "un-expecting" but I would feel a little better if it only happened to Clots.

 

[Ron]

Doable i think something like 1 in a 100, it would defiantly add a little suprise every now and then.

Btw you get this idea from resident evil.