I'm not sure if it can be requested, since it can lead to major change in both gameplay and code, but I would really love if any sound made during alien/civilian turn could lead to some amount of information.
Example: soldier is in room with three doors, crouching directly in front of one of them. During alien turn There's distinctive sound of door opening/closing. Now, from the realistic point of view, soldier should be able to determine if this was one of the doors behind his back or another one behind the one he's in front of. Or even some door far away from him (in this case, however, he knows nothing about it).
The UFO Aftermath/Aftershock/Afterlight developers overpowered this feature in their product, allowing sound detection as alternative to sight, marking enemy within specific cell of the terrain grid. I have no clear idea about how to make it both useful and not too powerful, but without it there's no real reason for any sounds to happen if nobody "sees" them.
Regardless, thanks for the great game!