A few new special infected
The Pyro: A zombie who always on fire and can breath it at short-medium range. If you trick him into water the fire gets put out but he can still breath flames, so molotovs and burn tactics wont work cause then you'd just re-ignite him. Imagine the panic this can cause to players, you get to the safe room while being chased by the hordes and one of these nasty basterds sets your ass on fire. Also playing as it you can have a great laugh randomly burning other infected.
The Spitter: He who wears no shoes! Can crawl up walls like a gecko and spits blinding stuff in your face from a long distance.
Bombers: Zombies that explode violenlty when killed or if the target gets close enough, destroying everything nearby.
Bring the Shrieker back and make infected animals.
And also three new levels.
Viva Lost Vegas: Casino of the dead! Run around blasting zombies in the neon lit streets near the start of the infection in an attempt to escape the dying city of no sleep. You get to drive an amoured up limo to get thru zombie infested streets and as a Tank you can throw pool tables at the survivors while the Hunter plays the House Of The Dead at the arcade.
Zombie Mountain: Survivors die while snow flakes settle upon red stained snow. Flee thru the snowy forests and into a mining complex to escape into areas that were less populated before the infestation. Dinomyte is plenty and the zombies are still flowing in full force while survivers run thru tunnels, use diggers and other minning objects to fend of the swarms.
Junk Fort: The Fort must not fall! The survivors have reached an abandend makeshift millitary fort and now the zombies are coming! Defend the fort until the rescue copter comes and shoot the infultrating shreikers to help stop the siege! The infected come in great waves from every direction. Stand on the great wall or towers and shoot the hordes from on high or defend the gates at ground level. With a large fully destructable segment of the great wall and other walls and gates, you must kill the tanks and Bombers before they make an opening! Team work between all 16 players is recomended in this epic stand off.
I hope those ideas are good ones.