make a gun game with really randomized properties, for example, sometimes the gun will shoot at you instead, or it will have insanely high spread, or even it just dissolves out of your hand
How about a 2Dish map where everyone is a cube soldier. There are two teams, one from the far left side and one from the far right side. The land is a rectangle and the camera is looking downward onto the map. The further you progress to the other side, the more one-time abilities you unlock like flying a helicopter with a machine gun inside for a few seconds.