This is probably going to just echo a lot of what other people have already said, or even things I've already said previously, but here goes.
1: Please for the love of all that is good, don't let cantrips create surfaces with some preparation going into it. For example, if I want to set an area on fire with a Firebolt cantrip, I should need to put something flammable like oil in that area first, not just be able to spam enough firebolts each turn to make that area into a giant lake of fire. Ice shouldn't just magically appear unless it has something like water to freeze. And please, I am begging you here Larian, get rid of Acid Splash reducing AC along with any other spells or abilities you've rewritten to reduce AC, it's seriously OP especially when combined with the almost constant high ground advantage.
2: As I saw someone above me say (huge +1 to you for that override367) don't give us actual Advantage on things just for being on the high ground. It's seriously overpowered, especially in a world where it seems harder to not have the high ground than to have it. Being higher than your opponent already has considerable advantages just from being able to get an angle on them when they're behind cover sometimes, making it harder for the ones on the low ground to reposition to an area you can't hit them from the high ground, and making it harder for melee characters to cross the area to reach you before they get slaughtered. This worked out ok in DOS2 because even melee characters had access to abilities that basically let them teleport up to the high ground instantly, but D&D doesn't and shouldn't have that. Another advantage to having the high ground, assuming a "fog of war" mechanic comes to the game as many people seem to be asking for, would be that you could see more of the battlefield.
3: Custom character backstory. I know there was a podcast where one of the devs said that the custom PC's would have just as much detail as the Origin characters, but there's a small problem with this. You guys decided my characters backstory for me? Nuh uh, nope, no thanks. If I wanted to play as a character backstory already completely written beforehand, I would play one of the Origin characters who seem to have far flashier backgrounds anyway. I want at least a little control over my characters background. You can even do it via a slightly more robust "tag" system like DOS2 had, just let me choose things like where my character is originally from, a background of some kind (blacksmith, sailor, etc etc), things like that. If my "custom" characters already has an entirely prewritten background to them that I didn't have any input on, how is this character custom to me? All I did was change the hair and eyes, everything else about them will always stay the same. I wanna play a Drow on all of my playthroughs then I have to put up with the exact same backstory every single time? That isn't a custom character, it's an Origin character that I can give a mohawk to.
4: The monsters need to, at least for the most part, stick to the PHB and Monster Manual. You've broken the entire flow of D&D with your rewrites by this point Larian, but the monsters too? Come on. I can get past the monsters by listed by levels instead of challenge ratings, but create them as if they are monsters, not PC's. The Gnolls are simply Gnolls with a few variants that give some different abilities, not Level 4 Gnoll Warrior God or something, they shouldn't get to attack 3 times in a single action and then follow that up with a bonus action bite attack. That's 4 attacks at level 4 which is more than my actual Warrior character can pull off at the same level! Also, I know the Gnoll Warlord we come across is supposed to be some kind of bigshot, but the weapon it uses is stupidly broken. 17 bludgeoning damage with another 47 psychic damage from a level 5 enemy? That's enough damage to put a level 6 Warrior (at least) in the ground. I get that it's supposed to be a big shot boss fight, but seriously tone it down. It's supposed to be a threat, not a freaking massacre.
5: We need to receive XP for more than just killing things, DOS2 had this problem as well. The only way to get XP is to kill enemies, so once you were done with everyone in Fort Joy what did you do? You murdered every last one of them of course, because you can't fall behind in levels or you'll end up getting stuck later down the road. BG3 now has the same problem, no matter how clever you are and how much combat you can manage to avoid through either dialogue or sneaking past to get what you want without being seen, you'll have to come back and kill them anyway because otherwise you'll never be able to progress since you'll be stuck at low levels if you don't.
6: Resting mechanics need a bit of tweaking, we shouldn't be able to take a risk free long rest in the middle of a dungeon with enemies on the other side of a door right after we just murdered their friends and cause a lot of noise doing it. And the fact that you just magically teleport back to your camp and then reappear back in the dungeon afterwards? Give us different environments depending on where we were when we hit the long rest button, or at the very least make it feel like we actually are camping out in the middle of the forest where plenty of danger lurks with some random encounter chances. Seriously, I know for a fact this place is crawling with goblins but not a single goblin or a pack of wolves or something stumbles across my camp? Cmon now. Also, give us at least 2 short rests, only having 1 short rest before we have to do a long rest is a serious pain for warlocks and others with short rest ability cooldowns.
7:Give us an option to hide helmets on a per character basis. My warrior looks really good in their steel helmet, but the leather helmet on my warlock looks dreadful.
8:Please either reduce the number of containers, make most of them into simple set pieces that cannot be looted, or at least put something into most of them so they feel like they serve some purpose beyond just being a bunch of empty barrels for me to spend the next 5 minutes searching only to come up empty handed.
9: Jumping/Disengaging need to be made into their own separate things. If I want to disengage from an enemy and run further from them, I should be able to do that without jumping around like a frog on meth. If I want to try jumping up somewhere and don't disengage first, then I should provoke an Attack of Opportunity. Jumping should be movement based and not take an action or bonus action, and disengaging needs to be an action since being able to use a bonus action to disengage is part of what makes the rogue class different and unique.
10: Holding actions, this would most likely need to be limited since it is a video game and you can't specify what you want to trigger it, but give us the option to select and attack and then hold it until an enemy moves within range of that attack. Basically it would be treated like the "overwatch" action from XCOM or something, so if my wizard can't cast a spell at any enemies from his current location, he can at least hold a spell to use for when they move into range.
11: Skill modifiers are supposed to be added after the roll, not deducted from the DC when doing skill checks and such. If I have a +5 in charisma and roll a persuasion check with a DC of say 6, and I roll a 1 on it then it is not supposed to be an auto fail, it should then add my +5 CHA modifier to that 1 which makes it a 6, meaning I pass the skill check.
12: Inventory clutter. We need a "sort all" button of some kind for better inventory management, I'm so sick and tired of having to click and drag everything in my inventory to sort it when a simple click of a button is something that is completely doable in games today. Also some way of reducing the huge clutter is making items more stackable, currently it seems like most of the herbs/plants don't stack even if they're the same exact thing for some reason. And keys as well, we need either some kind of key ring that all our keys get added to automatically, or keys simply need to be removed from the inventory automatically after use. All the keys I've come across in game so far seem to be a one use deal, but I'm not sure which ones go to what item and if I've used that key yet or not. Simply getting rid of the one off keys we've used automatically would reduce clutter by so much.
13: Starting equipment needs to be selectable based off of the class/subclass we choose at the start, the way it is in the PHB. My warlock should have the choice of starting with a light crossbow and 20 bolts for it or a simple weapon of my choice, along with my leather armor and dagger. Warriors should be able to choose to start with either chain mail or leather armor, a martial weapon and a shield or 2 martial weapons, etc etc. This yet again goes into the feeling of it not really being my own custom character, but simply an Origin character that I can rename and change the looks of.
14: Lockpicking should be decided by a dice roll, not a progress bar. I should see a dice roll and know right then whether or not I successfully picked the lock, not have to wait for a progress bar before it tells me whether I failed or not. For a D&D game, our dice rolls sure don't seem to have much impact or even be visible in some things where they should be. I'm fine with not seeing a dice roll every single time an attack happens in combat, but out of combat dice rolling should be a bit more prominent.
15: Special arrows/throwables/spell casters needs to be toned down a bit. I get it, you want to add a little Divinity flavor to your game and that's fine, I'm all for that really. But instead of some Divinity flavor added to your D&D game, you made a Divinity game with a little bit of D&D flavor to it, which is not what I was told I would be buying. Nobody is saying to completely remove all Divinity qualities from the game, but they really need to be toned down. Not everything should create a surface, not every archer needs to be loaded down with acid arrows, and not every enemy encounter should include a spell caster. Goblins aren't exactly known for their spell casting abilities, so why do I keep running into hordes of goblins tossing spells around constantly? A horde of goblins rushing at you with swords and shooting arrows at you is already daunting enough, giving every group of goblins a magic caster as well is just plain overkill, not to mention stupid. Some archers should have a small amount of special arrows, not most archers with lots of them. It doesn't make them feel special if they're all over the place. I should find an acid arrow and want to save it for a tough situation because it's a rare item, not just use it right away cause I know I'll get at least 3 more within 30 minutes. Same with throwables like acid vials and alchemists fire.
16:The spell UI needs a bit of an overhaul, especially in terms of casting spells at a higher level. Warlock has the luxury of all their spells and spell slots simply being upgraded automatically and not being able to cast the lower level versions of those spells at all anymore. But all the other magic casters have to deal with massive amounts of clutter caused by them now being able to cast spells at 1st or 2nd level, which creates a second icon on the hotbar. I'm terrified to see what the hotbar will look like when 3rd and 4th level spells come into play. Once you gain the ability to cast spells at a higher level, it should include a pop out menu like the Find Familiar and Hex spells do, where a box pops out above the hotbar with a selection to choose from. So when you select Magic Missiles, that same box should appear with the options for you to choose what level you want to cast it at.
17: Passive checks really need to be looked at. A passive perception check isn't a dice roll, it's a set of numbers on your character sheet. If my character makes a passive perception check while wandering around the world, it shouldn't roll a die to determine if I spot something or not, it should take my passive perception number and apply it to the DC of the passive perception check. Then I either pass or fail based on that number. I shouldn't be able to fail a passive perception check, then reload the save and pass it on the next go. I either pass it or fail it, no dice rolling and random chance involved.
18: Dodge action needs to be added, as does cover mechanics. Adding the cover mechanics would also make having high ground more useful even without it giving Advantage. Say there's a rock blocking half my characters body from the archers, I now have half cover which gives me a +2 to my AC and DEX saving throws. Now if the archers can get up higher so that it negates the cover, that gives them a reason to actually gain the high ground without giving them Advantage on rolls.
19: Skill checks in conversation need to be divided up between the stats a little more. Intimidation is technically a Charisma roll, but making it a Strength check would make high STR characters actually useful in conversations, rather than their only use being to hit things really hard. My party shouldn't have to rely on my warlock to do all the talking just because their class dictates they need a high Charisma because of their spell casting.
20: The amount of Auto Saves is way too low, I'm more likely to just drop the game for the rest of the day if I die and have to replay through the last hour and a half of things I did. Either make the auto save happen more often, give an option in the setting to adjust how often the game auto saves, or cause auto saves on short/long rests.
21: The final thing I can come up with at the moment is really just the the camera and pathing need some adjusting, trying to navigate through buildings is tedious when the camera doesn't want to give me a decent look inside the house, and when I click to move somewhere in the house the pathing shouldn't try to put me on the roof. Also been having an issue with my PC jumping off of ledges like some kind of lemming when they are literally standing next to the ladder to get down, and all the companions use the ladder to get down just fine. Maybe my PC is just dumb though, wouldn't know since her entire backstory is unknown to me.