Feel free to add a comment for any other ideas or if you simply just have a suggestion. Edit
First off, i'm sure most of Left 4 Dead players notice you are the only special infected while playing Versus mode. (Splitscreen) Edit
What i'm saying is, that while you are playing the role as a special infected, you should have some friends (although AI controlled).
They should be smart to a certain degree, knowing when they are in the line of sight of the survivors and trying to avoid being seen, and knowing to escape after attempting a attack.
AI controlled survivors are surprisingly smart in some cases. Knowing when to shove a boomer, sneaking around a witch, meleeing you from a smoker, or running from a charging tank.
AI special infected on the other hand should be just as smart as in, a boomer knowing when to hide and wait, or when to attack, a smoker running away waiting for his special attack to recharge, or a hunter leaping out of danger when being shot at.
^Your AI special infected friends act the same way they would when your playing in campaign mode.
Well, 4, just like in the first one, but with more special infected to be, certain special infected will spawn according to whats going on. (Example: Finale; Bunch of corner campers just spamming melee, and a Hunter dies, it is a high chance that a Spitter will spawn to break them up)
Well, it could start off just like in the first one (1 Smoker, 1 Boomer, and 2 Hunters), with time progressing into the level, Chargers, Spitters, and Jockeys will start spawning. Just to throw a mix of infected in there.
If you read this, I thank you for taking the time. Sadly, I don't know how to contact VALVe or email them any idea's of mine. If someone out there can do this favor for me, that would be much appreciated. But, please do not take this idea as your own...