I know this idea would involve a huge amount of work, but I am curious how if it has been suggested and how it is received.
The basic idea is to give the player and monsters a field of view(FOV) similar to the original doom. Your LOS would be a 120 degree wedge centered on the direction you are facing. This also implies a change of player control simailr to that of doom i.e. turn left/turn right, strafe left/right, forward/backward, and perhaps a turn 180 degrees control.
It would also be necessary to have a hit indicator, an arrow that points in the general direction(8 compass points) from which you were hit, or just points center, left and right, like the original doom.
Monsters would not be able to see anything outside their FOV, but for simplicity they would have a range-based chance of detecting the player outside their FOV, and they would always detect a player after he uses a loud weapon.
edit: Okay, okay. It was a very stupid idea that would serve only to slow down gameplay.