There are simple fixes that are easily implemeted into the game. None of what I am suggesting is impossible to do with the engine they have.
Make or break changes that need to be implimented or the game will not even be close to a faithful representation of DnD
1) Backstab/facing- remove the facing aspect and make it from stealth or if target is flanked.
2) Barrelmancy - Sorry, but wine is not explosive or particularly flamable and should not catch fire at all. Could it be used for difficult terrain, yes, but not flaming death. Grease should have to be targeted specifically in order to catch fire. Limit the number to what would be actually reasonable and in places where it would make sence. Furthermore, characters should not be able to transfer items once in combat unless they use an action to do so and are in melee range of the character they wish to trade with. This also gets rid of trading healing pots in battle.
3) Jump/Disengage - In the players handbook, disengage is a full action. It just allows you to disengage from an attacker so you can move away from them without suffering an attack of opprotunity. Jump is simply a movement action, if you go past someones threat range they should get an attack of opprotunity just as if they had moved through your threat range. Jumping should also break your stealth.
4) Height advantage - High ground should never give avantage. I would reccommend the simple +1 or +2 to attack from high ground and possible range extension. Also no disadvantage to shooting from low ground. Again a simple -1 or -2 would be fair for showing possible cover
5) Dodge - Add this full action into the game. It is an essential part of the core rules.
6) HP bloat - This may be a necessary evil since missing most of the time is not fun at early levels. My solution would be a tiny increase to mage spell damage to compensate.
7) Mage cantrips - Fire bolt, acid splash, and ray of frost needs the surface functions removed and their damage set to what is in the rules. Acid splash simply does 1d6 damage to one or two targets within 5 feet of eachother, not create a persistent acid puddle, Ray of frost simply does 1d8 damage and slows the target by 10ft of movement, not create a persistent area of ice and freeze water to knock things prone, Fire bolt does simply 1d10 damage to a target, not set them on fire and a persistent fire effect below them. It even specifically states it cannot be used to set things on fire that are worn or carried by the target. These three spell changes are my biggest problems so far, these three spells are what gives this game the feeling of being more Divinity than BG. Since these are what are lending to the Barrelmancy/surface warfare part of the game.
8) Monters being faithful to the rules - Minotaur and hook horror frogs that just jump around every turn is plain silly and has no ground it the rule book at all. 4th lvl fighters do not get 2 attack every round like the two 4th lvl duergar in the underdark. If those two duergar were ment to be 5th lvl instead and the 4th lvl tag for them is a bug, then that point is moot. There is most likely many more examples of this.
9) Push - This is an attack action in the rules and needs to be one in the game as well.
More nitpicky stuff that isn't make or break for me
1) Resting - limit how often you can take long rests
2) Waypoints - This has absolutly no grounding in Forgotten Realms lore at all. There are ancient portals in the realm, but they are very few and far between, and most no longer work. I put this under nit picking, because I know modern gamers are a lazy bunch and don't like spending 5 minutes to walk around a map and just want to teleport everywhere so I have no illusion that this will be changed. Just sad that a world I have known for so long is getting it's lore gutted like this for laziness.