Well, a severance mod would be nice or at least rag dolls A more powerfull headshot (because it is a headshot). The menu should resize when I the resolution is changed. Maybe zombie birds attack when staying to long at the same area.

EDIT: Well I played alot of zombie shooters on newgrounds. There are some really bad zombie defend games and some very good. This is what the good ones makes them good:
- Game is split into waves. Each wave becomes harder.
- between the waves the hero can buy weapons, upgrades, tools etc (a good balance of costs and usefulness is here extremly important)
- each almost each wave new kinds of zombies appear and more zombies appear
- the breaks between zombie appearing must be short. If zomtimes no zombie comes for 10 seconds and then again it becomes boring.
- usually the player has to protect something or a target (protect a house which takes damage if the zombies come to close, rescue a hot big breasted chick, survive till next dawn(?) (sunrise))
- well not all of these hints are really usefull for this game, but it may gives you some ideas for spicing up the game.

For example to fresh up the game, the player could get objectives to solve so he is not camping to much at one place and can make bonuspoints. For example, close door xyz, get to gate aaa and get letter x, kill the green zombie, take no damage for 10 seconds (extra fast zombies) to get bonuspoints, push the red button, destroy the evil machine, fight miniboss zombie xavbla etc...