ok these ideas are something like in 1.6 but i think you guys will like it

Predator mode

On a chance of 100 in a 100000 a random human will turn into a "survivor" and will be glowing blue and will have a gun that deals 300 damage per shot (but it is limited ammo though you have a pistol)
then to kill the survivor as zombie you will need to hit him 10-15 times
when this happens the survivor goes invisible for 20 seconds unable to shoot his guns and has zombie protection (so he can find a spot)

Nemesis mode


On a chance of 100 in a 100000 a random human will turn into a "nemesis" and will be glowing red and will kill humans in 1 hit and has 10 thousand hp , but when this happens all the humans near him will get pushed away for the first 5 seconds of this happening

OMGWTF mode


On a 500 in a 100000 chance the map will be filled with green smoke for 5 seconds then all the terroists will turn into zombies and one of them will be a nemesis and the counter terroists team will stay humans but one of them will be a predator (the predator should be used to fight the nemesis as his weapon will deal extra damage on the nemesis)

Antidote

Zombies will be able to buy an antidote for 4 credits turning them into a humans with zombie protection