Circling back to the discussion in general... as this did give me some fresh ideas.
The game-state probably would be healthier if:
Low-ground disadvantage is removed (it really doesn't make sense, bows were used to hunt birds as well as warfare).
High-ground advantage only applies if no enemy is within 35 feet & the character is at least 10 feet higher in elevation. (most enemies can move 30 feet in a round).
With these changes, high ground advantage would be implying that you could safely posture for a focused shot. And you may get hit with an arrow for leaving the character exposed.
As long as backstab is tied to melee dex weapons I think any solution could work, whether it be a feat or just base-game.
But this will still devalue a lot of things that grants advantage in DnD.
What happen today ? Highground and you does not care being targetted, with the disadvantage you're god. What will happen if advantage for highground is kept ? Highground but each turn we'll go away of the cliff to break the ennemy's lign of sight. New tedious tactics.
Whatever : still running for highground because it gives you the best and usually necessary bonus not to miss too often (even if your suggestion is obviously less broken than now).
According to me the real question is "If the dices have been changed twice because players missing attacks might be anti-fun, then why isn't the ennemy's AC reduced?" That's the only goal of ennemie's AC. Lower AC wouldn't have any consequences except that we'll miss less often. They have to know what they want : if missing is the main problem then they could just reduce the AC... Problem solved. Am I wrong ? Is there something I'm not thinking about related to AC ?
- Advantages in BG3 devalue conditions, spells, features and synergies betxween characters for the sake of "missing less often because missing in a video game is boring" - which is true but my perception is that "missing less often" is not at all what advantage is about. Advantage is a reward for smart and tactical moves and not something required at each turn because your DM is bad at balancing combats and/or to compensate the lack of tools to increase the %to hit in DnD. Blinded, paralyzed, petrified, restrained and prone are not really worth it if half your characters can have easy advantages.
Advantage/disadvantage should stay an "advanced" technique. Various flat bonuses would also help with the "missing too often" issue and will give us new tools to improve our %to hit without breaking anything related to the the advantage mechanic. It would also allow us to have an (additionnal) advantage (on top of it) through any DnD possibilities. It shouldn't be necessary at this "normal" difficulty levels but may be at higher and whatever the difficulty it means more choices to increase our %to hit, more meaningfull synergies, more meaningfull tactics and way more depth to combats.
- Giving the best bonuses just by going higher (or going behind) will not allow anything interresting to play with the %to hit at higher difficulty levels. What will we have to fight better AC's ennemy's ? Bless for an additionnal +1d4... definitely but what else ? Proficiency bonuses, +"x" items; better modifyers... yeah yeah that's very DnD but DnD doesn't have "higher difficulty levels". Which tools will we be able to increase our %to hit over ennemies with an increased AC if the best bonus is easy to have and required in the normal difficulty level "not to miss too much" ? Well... nothing...