What are the main roadblock for implementing a graphic visibility system? Is it within the game engine itself that limits a visibility system, or is it difficult to implement in a Quake game? I've seen Quake games where weapon fire illuminates the surroundings, but come to think of it I've never seen a Quake game where you can shine a flashlight around (unless its the modern engine).
That case is complicated. And flashlight and visibility system are two different things. For example, flashlight was implemented in a Half-Life, but it used a hack (virtual spot light). Doom III uses a true flashlight, and it is still based on the similar engine architecture.
Visibilty is the other thing, engine already does it for many things, but
rendering of it is not supported by the current engine, and WILL requre a somewhat different rendering approach. To be exact, a some sort of "visibility shadow" should be calculated, and the engine (currently) does not have any provisions for that. Alas