Background
In the Player's Handbook, it is specified that Advantage and Disadvange sources effectively "cancel out" regardless of how many there are of each:
If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20. This is true even if multiple circumstances impose disadvantage and only one grants advantage or vice versa. In such a situation, you have neither advantage nor disadvantage.
Intuitively, this rule makes sense, but it results in some very bizarre consequences mechanically, especially where multiple Disadvantage sources (which is far more common than having multiple Advantage sources) are concerned.
Take, for example, creature A firing a Longbow from 550' away at their (aware and engaged in combat) target, creature B:
- From being 550' away, creature A has a source of Disadvantage
- THEREFORE, creature A has disadvantage on their Attack Roll
Now consider the scenario where this same creature A first steps into an area of thick Fog before attempting their shot, and is therefore no longer able to see their target (and vice-versa; creature B can no longer see creature A). They're still aware of creature B's location (because creature B has not yet moved), but nonetheless they cannot see them.
- From being 550' away, creature A has a source of Disadvantage
- From (creature A) being unable to see their target, creature A has a source of Disadvantage
- From (creature B) being unable to see their attacker, creature A has a source of Advantage
- THEREFORE, creature A has neither Advantage nor Disadvantage on their Attack Roll (???)
Do you see the problem? By doing something that, by all objective logic, should have made that shot more difficult by obscuring vision to their target, they have instead made their shot easier; blinding themselves caused a significant boost to their shot accuracy.
My Proposal
To fix this issue, I'm proposing allowing Multiple Disadvantage sources to still take precedence over a lesser number of Advantage sources. This rule would not be symmetrical; it would not be possible to gain Advantage while having any number of Disadvantage sources. This is my proposed [re-]wording of that paragraph:
Variant-Multiple sources of Advantage or Disadvantage. If circumstances cause a roll to have both advantage and disadvantage, tally up the sources of Advantage and Disadvantage conferred upon that roll. If the number of Disadvantage sources exceeds the number of Advantage sources, the roll has Disadvantage. Otherwise, the roll is made without Advantage or Disadvantage; having an excess of Advantage sources will not result in a roll that has Advantage.
The intent of this rule is to make it less feasible for characters to make weird "trick shots" like what I've outlined above, where a course of action that should logically make an action more difficult instead makes it easier.
Are there common/notable situations where this ruling could backfire instead? If so, what are those circumstances?
Why not Symmetrical?
The short version is that Advantage, to me, represents circumstances where a character is especially able to perform whatever task they are trying to perform, be it dodging the blast of a Fireball, or successfully making a shot with an Arrow. If there are circumstances that are interfering with their ability to do so, even if they are receiving a lot of bonuses to their capabilities, those interferences should generally negate whatever ease they were otherwise experiencing.
Best Answer
It complicates what is intentionally simple.
One of the aims of the advantage/disadvantage system is to remove the payoff for "bonus-scrounging". It isn't desirable to have your players constantly trying to find one more reason to get a little plus in their column; that has historically led to a lot of friction, book-diving, and absurd arguments to realism ("I'm standing on the table so I have a high ground bonus against him!"). By making only one advantage 'count', your players are more likely to focus their attention on finding one strong narrative (or mechanical) reason they should have advantage, and then stop looking and roll the dice already.
Your proposed rule partially brings us back to that place -- when there are X disadvantages in play, the players are suddenly strongly incentivized to try to find at least X advantages to balance it back out. Instead of looking for one decent source of advantage, they're hunting for enough to balance things out, and you're back to jumping on tables.
But you may be trying to fix the wrong problem.
From your question, I'm not sure whether you actually have a problem with the advantage system as a whole, or if you're really just seeing a problem with a very specific sort of scenario, and aiming a little too broadly with your fix.
Your example scenario with the fog cloud does highlight an issue I've noticed myself -- I personally think the "unseen attacker" rules are a little hinky. But the problem is in the "unseen attacker" rules, not in the advantage system itself.
It would be easier and less intrusive to make a houserule that modifies the "unseen attacker" rule to say something along the lines of "When you can see a creature but it can't see you, you have advantage on attack rolls against it." Then going from a disadvantageous shot to a double-blind scenario doesn't improve your odds; if you're both blinded to each other, such as by a fog cloud, you both attack at disadvantage, which makes sense both narratively and logically.
That fix would solve the issue you've identified but wouldn't require making the overall system more complicated -- if that's the only thing bothering you.