Zombocalypse Solid
My personal preference is the top-down zombie shoot-em-up, but let's continue.
This game is astounding enough in portraying the zombie apocalypse, that it really doesn't need any "upgrades" or anything. The only think I can suggest is that the melee be switched to a baseball bat, and that it always be secondary. The minigun is an issue with the runners, which is why the secondary weapon being a baseball bat and always being available is my only concern.