I think this idea has more potential and is more than just a small suggestion.
In this moment there is no fog of war in OW. The Overwatch can really see everything. Every rebel tactic can be recognized early and this is not realistic. The minimap is also only available per consol commands.
Now imagine we would restrict the view of the overwatch. It can still see the whole map (the Overwatch knows the city which has to be overseen, of course) but not enemy units (maybe props, too?). Only the units it can control got line of sight and let the fog of war dissappear around or in the direction they look. Two units would really make sense: city scanners and cameras (as trap based on maps) which would be a mobile and a stationary opportunity to uncover the FoW. If it is not too complicated, I suggest to uncover only the area which the unit can really see. Hence there is a fog of war behind and near the unit.
Sounds too difficult for the Overwatch? I had also the idea to add noise detectors for the minimap like in Crysis. When there is loud noise like gunfire or explosives there will be a growing circle around the location of the noise on minimap.


Detected rebels could be red dots on the minimap. On the first contact a voice could tell you: "First encounter with rebels." Pressing space bar could jump to the last detection of a rebel.
This would really change the gameplay, for both sides. The Overwatch would have to scout with units and to monitor important areas using units. On the other side the rebels would have the opportunity to use sneaky tactics. More options, more atmosphere, more fun. :-)