I'd like to summarise some of the most favored Ideas in case TWI would like to take a peek at this thread anytime soon:
(separated into two groups)
Enemies
1)Dog-like enemy. Shorter than Clot, fast as Charging Gorefast.
2)Gigantic enemy? Was suggested a few times. Something that could charge and stun people. Massively big and slow, but no charge/rage.
3)Ranged enemy; Perhaps Biotic Soldier with small gun grafted to it?
4)Swarm; huge amount of small insects? Like the Scarabs in TimeSplitters3 I'm assuming. Weak to fire.
5)Zombie with multiple heads, making it harder for Sharpshooters to handle.
6)Flying enemy
7)Zombies with lasers on their foreheads; for the lulz
Effects
1)Poison; slowly kills until player heals/gets healed
2)Slow-down; slows players hit
3)Ranged fire/gun/biological attack; single player targetting. Can be blocked by other Zeds to even the odds.
4)Suicidal "explosion"
5)Bonus to nearby Zeds. Zed support ability of some kind.
6) Zed that can regen.
7)Zed that retreats when weakened? Would probably be best couple with either a ranged attack of Regen.
8) Hallucinogenic. One that would screw over your screen with a multitude of colours for a period of time.
I also just had the idea of a Zed whose sole purpose is to be repulsive. Nothing amazing damage-wise, but with a few abilities that are just plain wrong looking or sounding. The ability to disgust/annoy/sicken/scare players could be potentially great imho.