I'm not a D&D player - I don't like the system at all for Pen'n'Paper - but listening to Larian's marketing I was expecting a game attempting to be 95% D&D. Just diverging from it where it really clashed with the change in medium.
Still even not knowing the exact rules beyond reading up on classes, some youtube class reviews and listening to Critical Role, I can imediatly tell which parts of BG3 are not based on D&D and that for a very sad reason: They obviously break the balance of the game - cantrips imbalance, surface damage, amount of grenades and special arrors, food, bonus action economy,...
This is why I would argue to stick closer to the rules. They are not improving on them (like the changes for the ranger class), but very often disturbing the balance of the gameplay (prime example rogues). Some might claim that D&D is too simplistic because their move+action+bonus system isn't deep, but neither do single chest pieces offer flexible or interesting movements - the sum of all chest pieces is far greater than the value of individual units and the same can be said of a full party's vs single characters' actions in a round. Again, I myself am not at all a fan of D&D and still I see that it offers enough depth for an interesting game.
On the other hand there is pretty much nothing in BG3 that at this stage couldn't be changed very easily to get back to a far more balanced D&D 5e version. Its not re-inventing mechanics, its just assigning proper action costs to current bonus actions, giving those cheaper ones back to rogues, reducing food and surface affects and a couple of tweaks to spells/skills. None of that - even if it requires re-balancing levels - is that much work that it couldn't be done within a few months for the whole game and its pretty much why we have only 6 classes and just level 4 with no release date around the corner. So I am not worried at all.
My suggestion for Larian would be go back to 5e to reduce the amount of balancing you need and only then start tweaking and diverting from it where problems arise. Starting from this 'middleground' between DOS and D&D 5e (which is BS, its far more 5e, but people love to jump the 'Its DOS-bandwagon' due to obvious problems) you are not doing yourself too many favors when it comes to amount of work ahead of you.
