Haha first of all, great name for a game, that P.A.N.demic and P.A.N.ic thing is clever =P I love the idea of this game, A simple, bloody, yet effective, shooter
And I believe I know where the direction you want to go in this game is... You want it to make it a laid back fun hack'n'slash game (but with guns instead) similar to Left 4 Dead. Left 4 Dead has very simple gameplay, but still people cannot put it down. The reason for that is, the blood, and the speed of the gameplay. If you wish to do this, add a lot more blood, when a zombie dies, try to make it so blood spurts come out (and make it so there's different blood spurts that can shoot out for effectiveness), make the zombies die quicker and add a lot more of them, this should give the sense of chaos, much like L4D =P As far as playing the overmind goes, it's badass, however, the special zombies should have alittle more twists to them (cept the webslinger, i think he's pretty much perfect) Make it so u dont just walk around with them, but u gotta play them in a special way. Perhaps, have the wailer have a short timed and low cd windwalk, lasting only like 2 sec, or better yet, make it so she has like shadowmeld, so she becomes a landmine of sorts. stuff like that. That being said, maybe you could add more players to the mix (some play special zombies, while another can play overmind)
All in all, great map and amazing idea, just needs that fast paced, bloodiness that L4D has =P