A controller zombie might work. Not a zombie that could call up huge hordes, but he'd get a steady trickle of infected, and he could save up to about, say, ten or twelve, then send them in a click at a time at a designated target. The controller could run as fast as normal infected, and shove people over if he attacks from melee. the Controller's purpose would be to add just a little extra pressure at important moments, or to distract a players attention while a hunter lines up a pounce or a boomer comes around a corner. Or he could send in some mooks to help a smoker lay on the damage. He wouldn't have enough zombies to do much by himself, and he'd be relatively weak, but he could do a little here and a little there to tip things over the edge.

A berserker zombie, that can run very fast and throw players around, but not very far. Something between a hunter and a tank, a bit tougher than the hunter, but without the tanks incredible damage and durability. The berserker would charge into the group behind a screen of infected and use his throw special and knockback normal attacks to disrupt the group.

Howler/Screamer - Pretty simple, the howler or screamer would have an of effect vocal attack that would disorient the players and that proc'd every 3-30 seconds, and could grab a player with their special attack and scream in their face, deafening them and disorienting them, while doing damage. The disorientation would wear off pretty quickly, but the deafness would last for a few minutes.

Sludge - More or less a boomer, but his puke and guts would glue you to the spot or slow you down.

Normal infect! Open up the opfor to more players and let them spawn in and normal, run of the mill infected! give them a 'rage' bar like the tank. once the players are in range they have to move towards them and try to attack or lose control of themselves and spawn as a new infected.

Laughing Man - More or less a different sort of witch. a tall, emaciated figure in shredded threadbare clothes, The laughing man laughs continuously, loudly, and dementedly, while wandering around a small area. Ifa player player looks directly at him he'll fix a gaze on them, paralyzing them in place, leaving them unable to move or shoot. He'll then walk towards them at a fairly slow pace, laughing maniacally all the while. If he get's close enough he'll cut the player down in one blow, like the witch, then proceed to rip them apart, still laughing and giggling psychotically. While the witch needs to be avoided then runs at the player rapidly, the laughing man cannot be looked at directly, and the first player to do so is paralyzed. To get around the laughing man you have to maneuver past him without looking at him, or gun him down, a risky proposition because he is just as dangerous as the witch if attacked.

Terror - A clawed monster that runs along on all fours, a terror will try to run up to a player, grab them, and exhale a cloud of gas into their face. The gas induces temporary irrational terror, and the player character runs screaming in a random direction. If they get away from other players and agressive zombies they'll hide in a corner and cry for a period of time. Zombies are much less agressive while the character is affected by terror, but once the effect wears off the zombies will attack. Again, a method of isolating the players. The terror zombie is fairly fast, but not hunter fast, and has a moderately strong punch attack.

Ghost - stark naked except for a few tatters, this zombies skin seems to change color and tone to match it's environment. While the effect has limits, the zombies skin will change from white to very dark grey in response to lighting conditions, and the skin tone can change from a pale blue to red to yellowish, depending. The zombie tries to ambush a player, then grabs them and runs away with them until it is out of sight, at which point it begins attacking.

Spook - Stands perfectly still in the dark, and can press up against walls or even cling to ceilings. When the spoke sees it's pray, it can rush very, very quickly towards the target and bowl them over, screaming all the while. Think of some of the critters in house on haunted hill. The spoke goes from not moving, to almost instantly being in your face, with the express intent of startling the player and forcing them to change their pants.

Trap Door Spider - You'd have to mess with the levels or something, but basically that monster that is always dragging people through holes in the wall or into airducts in monster movies. Think the scene in Alien III in the medlab. If the zombie successfully grabs a player and drags them into a manhole cover/air vent/under a car/under a desk/over an edge, the other players would see their kicking legs, hear screaming, and see blood. they'd have to 'e' key to drag the player out, at which point the zombie would charge and attack.

Echo - An Echo would confuse players sense of where their teammates are. The teammate highlights wouldn't work properly when an Echo is around, and the Echo would repeat anything other players said, but with the voice 'thrown' in an area around the Echo. The Echo's special would be calling for help in the voice of one of the players, which would cause that character's silhouette being attacked by one of the other specials to appear around or near the Echo. If someone came to look the echo could attack. somehow. with damages.