If you remember the ask a BB day thread, I asked once whether it is true that open shots are effectively guarded by team defense (rating). I didn't get an answer, but other folks have since come to believe that it is indeed so. In general, I agree with the direction of this suggestion.
However, the problem I see with your suggestion is that it would require rather extensive tinkering with the game engine. I'd suggest that they slash the defense by half, or even more (two thirds?).
So, I'll use D&D as an example here.
Your AC (armor class) is what it is, to protect against hits.
when you use cover:
A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.
A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle. The obstacle might be a portcullis, an arrow slit, or a thick tree trunk.
A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.
and then all the stuff is based off of rolls with a 1d20 die.
so i went that route, because a 1d20 die, isn't that much different than a math algorithm.
There should be differences in the defense based on how open (how covered) you are.