Idea Idea - Infect

(on phone so excuse the short post)
Essentially, Infected could have special unlockable abilities, for instance:
-Doubled armor
-Reduced loss in stamina
-an AoE effect that buffs other Infected/weakens the Survivors

Or even special enemy types such as those in Left for Dead 2:
-Chargers (can put on a massive burst of speed and knock down a survivor)
-Spitters (can spit a poisonous substance that damages and slows Survivors)
-Jockey (can leap onto a Survivors‘ back, can be shot or thrown off)
-Tank (a zombie with huge stature, damage, and an immense health pool, but quite slow)

Thoughts?