F-Stop (aka Portal 1.5) was / is.
Come up with a game pitch in the comments, if you'd like. Focus on the mechanics and what possible puzzles / levels would be, and for extra credit you can think about what the technical implementation would require.
You have an F-Stop gun that controls how much light comes into the world, or something. By taking "exposures" of different parts of a level, you expose it to different light levels. To sneak past a security camera, take a low exposure of the area so it can't see anything.
You can also turn on a Flash, to overexpose a dark area or something.
Surface materials play an important role: matte materials will behave, but glossy materials will reflect your Flash back at you -- so maybe in a dim area, you'll take a Flash exposure but it'll only illuminate the silver box in front of you, just like in real-life photography or something.
Maybe some levels will require you to setup light cards / lighting kits / light filters. You might have to reset your white point several times, etc.
Imagine: an FPS puzzle game that teaches you how to properly light photos!
I have no idea how you'd implement this though -- you'd need real-time radiosity and real-time caustics, or at least something that reasonably fakes both of them? Could Source handle it gracefully? Probably not.