Wow, well done.
Well, what can i say. Apart from OMFG! This is brilliant and addictive. I can see you have spent a lot of time making and coding a game like this. I few things i have thought of whilst blasting at those pesky white zombies:
1. A save button, cos it takes ages.
2. A mini map.
3. More weapons obviously.
4. A story mode where you go through a house and a forest and even a boat to try and get to safety. There can be some bosses that take movement, shooting and patience to beat. Some different zombies could be intoduced along with the red ones.
5. An online multiplayer mode where you can team up with mates or just someone else who doesnt have anyone else to play with.
6. Maybe some sort of weapon store or upgrade function other than the combo thing.
Thanks for making a great game and filling some time. Cheerz. KJR :)