I recently finished Alan Wake, a game in which the enemies are weakened by light. The player uses a flashlight, flares and flashbangs, and seeks out lights in the environment, turning on generators etc. to help defeat the Taken. But I just remembered an older game, The Darkness. Based on a comic book, the protagonist has several powers that he can only use in the dark, the player must often shoot out light bulbs and turn off light switches etc. to give him an edge over the enemies. In other words, similar concepts approached from different sides.
And then it occurred to me, why not take the same idea and put it in a Left 4 Dead kind of game? One team of players control humans trying to escape or transport cargo or find an artifact or protect civilians or some other mission that has them traveling across or defending an environment, while the other controls the Darklings trying to kill them. All using light-vs-dark mechanics. The environment should have light and dark areas, possibly with changing lighting conditions to make the combat more dynamic.
Darklings are strong and fast and have various powers, but are weak and slow and lose some powers in the light. Humans have weapons and flashlights; if a Darkling can catch them by surprise they have the edge but if a human catches them in the light they are at a disadvantage. Flashlights have less effect than larger environment lights etc. The game could use many players per side, or the Darklings have minor AI-controlled darklings for gameplay similar to Left 4 Dead.
There you go, free awesome idea. You're welcome, games industry.