Left 4 Dead is indeed a pretty cool game that doesn't require much to make it better. Having AI throw grenades and having a chance to be turned on death are good suggestions, although each carries with it a bit of a design challenge. (AI throwing grenades: what about AIs tossing molotovs and killing you? Characters getting turned: Boring for players whose characters are turned and are waiting around to respawn -- granted, if this was added, they must have thought of a way around it.)
My own suggestion for Left 4 Dead: Allow the director to send in more than one of the same kind of special Infected during Campaign games. Right now, you'll always encounter a maximum of one Smoker, one Boomer, and one Hunter at a time. Never two or three of a single one. (Note, I said campaign game - versus mode games this already happens. Also note I'm not recommending two or more tanks be sent in at once... not unless they add a "sadistic" difficulty mode.)
The main reason why this would be an interesting addition is that it adds a bit more randomness, and I think we'll all agree, flexibility to the randomness of how each game turns out is one of the coolest features of Left 4 Dead.
My own suggestion for Left 4 Dead: Allow the director to send in more than one of the same kind of special Infected during Campaign games. Right now, you'll always encounter a maximum of one Smoker, one Boomer, and one Hunter at a time. Never two or three of a single one. (Note, I said campaign game - versus mode games this already happens. Also note I'm not recommending two or more tanks be sent in at once... not unless they add a "sadistic" difficulty mode.)
The main reason why this would be an interesting addition is that it adds a bit more randomness, and I think we'll all agree, flexibility to the randomness of how each game turns out is one of the coolest features of Left 4 Dead.