What u say about this? Isn't it the greatest idea? Here some examples:
If you catch 5 zombies with frost nade you will get higher range of frost nade (that means u can use it on larger spaces)
When u kill 10 last humans you can get higer speed with any zombie class (e.g. 300 speed with any zombie class)
If u are human and u killed 3 zombies with HS in one round u will get higher damage
If u are human and u killed 1 zombie using only the pistol u will get a chance to freeze a zombie with any pistol ( 20% controlled by cvar) .... oh i forgot u must to kill that zombie in only 1 round
If u killed a zombie with knife u will get a hammer instead of knife and when u hit a zombie he will be pushed away
If u are zombie and u infected 5 humans in a single round u will get a infection bomb next round when u will be infected