i have played it and its alot of fun.
although i like the zombie mod for counter strike source more
this is what i would like to see:
special maps for zombie mode with lots of hide out places for the survivors(lol in css zombie mod often barricade myself in a small room with some furniture infront of the door and then when the zombies come you have to shoot the furniture or they will come in ,i often start screaming as a little school girl)LOL!!!!!!
any ways maps cannot be too big so maybe purely custom maps would be cool.
zombies need to have some extra power like beeing able to walk faster and way more hp.
special zombie skins.
maybe also zombie escape maps(this is when you have to escape from a certain area because its gonna be bombed for example) but not only that you also have zombies trying to zombifie you ,i play it alot and its rlly intense. <(^.^)>