It runs smoothly and the zombies are really good. It coud have a reward system for killing and surviving like getting better weapons or recieving a power-up if you survive a few waves without damage, different types of zombies would also be nice, background music, sounds and custom woud be good too.
Those are minor things and with time you will have everything done. The game is actually fun and i hope you can do all the things you plan :)
Hey, Gio, if you are reading this, greetings from pluto that played the game with you for a while :)