Well you have to remember that the reason behind the special infected was to induce teamwork and to punish lone wolves, so if any new infected were to be made, it would have to fit in with A. incapacitating survivors forcing other survivors to rescue them and B. fit in with the overall feel of the game.
For example, the screamer would have felt too out of place, a zombie in a straight jacket? I can understand why the attracting zombies bit went to the boomer because the screamer would have killed the overall tension of the game and the whole mutations in the zombie virus stuff.
So, we have pounced, constrict, blindess and attracting zombies, emo death witch and good old muscle.
Its hard to think of a good one to add that compliments the game rather than hinders.
Perhaps you could have something called a 'Spiker', if you have seen X man 3, theres a mutant who fights wolverine that can fire organic looking spikes, maybe you could have that who can fire spikes and pin a survivor to a wall or floor but has to be quite close to do it (no where near smoker ranger, maybe boomer vomit range)