I agree with those stating that the size and scope of this game in the current era likely won't be successful enough sticking to a fully faithful interpretation of the 5e rules. That said I do think there are some changes that do make sense that won't hurt their standard gamer fan and will address at least some of the concerns from the D&D and BG fans.
Most of these have been mentioned but I will also add my thoughts as someone who never really played the original BG series and someone who, while interested, has never played a full campaign of D&D (started one but the group fell apart.)
1: resting, for the love of god this is something that is stupid in it's current state. short rests are useless because you can simply long rest w/o consequence. Not only that the game actively encourages you to take long rests every 15 feet as over half the EA story plays out in camp. The amount of content I missed cause I thought long resting was something that should be done sparingly is staggering. This is especially weird because of the time-crunch the game appears to put on you (turning into a Mind Flayer and all that.) I thought I would be playing around spell slots trying to save them for good opportunities. Nah, just spam all the spell slots and take a long rest every fight... (honestly I thought Fallout 4 felt like it was rushing you to do the MSQ without detours but BG3 takes it to a whole other level.)
2: Reactions, not a lot to say about this other then it just doesn't work that well in it's current iteration.
3: Dipping, honestly I don't use this mechanic because I think it's tedious and adds nothing (except extra dmg) to gameplay. (coating weapons pre-fight in oil or poison could be a thing.) at least at that point me setting my sword on fire with a candle would at least make some semblance of sense.
4: Advantage, This is something I have thought about a fair bit from an outside perspective. On one hand I get it standing at someones flank would realistically give you an advantage on attacks, that's common sense. and in the Immortal words of Obi-Wan "It's over Anakin! I Have the high ground!.) High ground is also realistically advantageous. However, when playing I found this aspect a best tedious and at worst a pain in the ass. This was true in DOS2 as well and often takes me out of the immersion by repositioning my characters mid conversation (or before) into advantageous positions. in part because my ranger and mage would brilliantly be standing right next to the enemy at the start of the fight. (sometimes this would make sense like if we got ambushed but other times why is my archer standing face to face with a fighter when objectively the odds of a fight breaking out are high.) you could get around this is dos2 with shortish CD movement abilities so it wasn't that big a deal but in BG3 it uses a spell slot which you would ideally want to save. That's to say nothing about the fact that rogues/rangers have no such ability at least early on. With all that taken into account I feel elevation plays to big a roll in advantage especially with how the system works currently.
5: Action Economy, let's start by saying dash is largely awful.... Jump fills a similar roll and often does it better as it only uses a bonus action. This could be at least slightly solved if jumps range was limited by your movement per-turn. or used a full action. Only rogues (and maybe rangers?) should have sneak/stealth as a bonus action. (AI still needs to be less retarded.) for all other classes it should be a full action.
6: this is more of a personal preference after trying out solasta but I would like to see my rolls on screen (and not just the 20s and 1s) I noticed when playing solasta I was less frustrated when I missed a target when I was able to see the rolls. (this can maybe be a toggle-able option.)
Honestly, there are probably other changes that wouldn't effect the average CRPG player but would help alleviate some of the issues D&D and BG fans have but these are the things I can think of ATM.
Personally I like the game more or less as is. (with a few exceptions like resting being required for half the story plot.) But many of the things I mentioned I wouldn't have an issue if they did get changed to help alleviate the issues other fans have.