The game graphics and mechanics look great. If the game is level based then you can just make each level harder and every 5th level make it some boss type a bigger verio n of the zombies you have. If its going to be endless the easiest way I know so you don't have to code so much is set the zombie speed to increase when your score reaches a certain number. Then set a system timer to release a boos or bigger zombie type every 60 seconds or so.
Or if you want to go nuts and have some crazy fun with it make a flying headless zombie with this code
System(every 3.0 seconds) FlyingZombieHead(set angle towards (Player.x,Player.y)
That will throw players for a loop haha.