Few thing i would like to see:
Different kinds of zombies (as said before)
More survival stuff - you need to find food in shops etc to survive
Most cars have low fuel - you need to find fuel
Faster zombies so it´s more challenge
More story - f.ex if you enter you can watch how all this happened
Different spawning types - f.ex i would like to see army troops - thugs (which would rob food etc from others) - scientists (which are in a bunker with some challenge of doing anti-zombie medicine)
If you shoot from a gun, the other zombies will take notice and run to you (makes killing with a silencer or bat much more usable)
Something that will get people to join groups - even a group system which will share the resources they will find and groups will find stuff faster if they are big (so people will join groups)
If you are attacked by zombie and "killed" you won´t lie.. you will become one of em
Might be unrealistic idea´s but i would love to see that.