[Imp]

First off an assumption.
I assume a hindrance level (to vision) is placed across the map based on Global Vision setting. The lower the setting the higher the hindrance resulting in units not seeing so well as the Global Level is lowered.
If this is the case how hard would it be to have a second hindrance map that adds a random amount of hindrance each turn.
It would have more effect on lower global levels and maps with lots of hindrance terrain.
Reflects changes in weather.

How I think it would work.
Global vis 12
Slight increase in hindrance - some units that were visible through hinderances wont be.
Bigger increase units can now only see 11 hexes in clear terrain before its blocked.

By this I mean Global set to 12 would produce a hinderance map that would randomly reduce sight by by say up to 3 hexes across clear terrain.
Set the average randomness to one below the global.
This would result in global setting of 12
Vision is normally 11 clear hexes but could be 9-12
No more parking your unit just outside visual range so this would help the AI