We (or at least, I) liked the idea of having zombie infestations across multiple eras. Here's a game that would take advantage of that setting. You control a zombie horde. Your means of controlling them is represented by an arrow that you may move around the edge of the screen and orient as you'd like. When you click, you unleash a stream of zombies upon the gamespace in the center of the screen. Somewhere in this gamespace there is a historical figure that you must try to zombify. Around him are buildings, people, obstacles, etc, that all influence the movement of your zombie horde. You must, almost in puzzle game fashion, figure out a way to overcome these obstacles to get to the person you want to zombify. Here's a quick mockup. Gray dots are zombies, purple squares seem to scare them away, X is their target.
This type of game may also lend itself well to the "play three games, design one" idea (see post about unicycle bears and zombie sharks). Each successive era would have a toolset of buildings and obstacles that provide more and more of a challenge (Angry cavemen < angry French peasants < angry man behind MG42 < Predator missiles, hut < mansion < palace < white house < army complex of the future). You would design levels with a drag and drop interface, with an allowance of a certain number of items. As you played further and/or as your levels were played and provided a challenge, you unlocked more eras and therefore could design more and more complicated/challenging levels.