From a discussion in another thread I thought of that idea of having a tool that let's you hear farther.
A sound Mirror concentrates sound from where it's pointing at. With the amount of clouds in this game it could be quite usefull to have a sense of direction where the enemy is. The tool only works if you focus it and the engines of the enemy are on. You still won't be able to mark him with that but something similiar with the crosshair of the rangefinder could work. The direction of the sound can be seen as long as somebody is using the tool.
Concept of the tool:
also I know we have drums, but in contrast this tool will allow you to pin down the direction of the enemy.