Well! I was actually going to suggest light levels in DoomRL at some point (though I was hoping to see implementation of secrets/enemy room triggers/better teleporters first :-(
But I wholeheartedly support this idea. In fact, now that I know more about how rooms are generated, I know it wouldn't be too hard to include this. There could be different light levels; normal (9 LOS), dim, (5-6) dark (3-4) pitch black (1-2). (Perhaps even the rare "extra bright" for extended vision.) Enemies, naturally, see perfectly fine in the dark. (Alternately, some enemies have night vision, some don't.)
In pitch black rooms (i.e., only 1-2 squares vision), in some cases the lights could flash every x turns, showing enemy locations. Also, whenever an enemy attacks, its position would be revealed in rooms of any darkness. The difficultly, naturally, would be in figuring out where it went afterwards. Perhaps the accuracy penalty could be adjusted in rooms with low light levels, as opposed to normal non-LOS shooting, to make it more difficult...
The chances of encountering a dark room would depend on the level types. i.e., in "city" level, the outside areas would always be at normal brightness, probably same for maze levels. Cave levels would always be at "dim" or below. I would envision this having the most impact on normal and "small room" levels....for some reason, it seems fitting that normal levels would almost always have rooms of varying light levels.
The idea of having light sources is also rad. We could finally find a use for the classic lamps/torches of Doom in DoomRL! Naturally they would only spawn in dark rooms. Of course, there could also be some level events centered around this...such as a "dark maze" level. (Gosh, that would be hellish.) In this instance, light sources would be spawned throughout the level. Perhaps they could even be seen at any distance, as long as they are in the LOS, also lighting up an area of 3-4 squares around them. (Since mazes have so many walls, long-distance spotting will be rare.) This would be a different and challenging way to play DoomRL.
Overall, I think this would make the game even more intense. Since our fanbase is probably only getting more skilled as time goes on, this would be a great way to ratchet up the difficultly without drastically changing any core elements. (i.e., getting rid of corner shooting or something.) Throw in some light-amp goggles and it would be perfect. (Maybe it would also justify blur sphere?) I would love to see this implemented.