I cant find anything wrong with it
Seriously. A well made game.
Now, how to improve it? Who knows.
But may I suggest: Multi-player over the net? If I can get 6 friends to each grab a gun and defend our ramshackle base... wow. And its a constant game, where you're either killing zombies or repairing the wall. the goal? survive for as long as possible.
But really. Superb game.