1. Display Grid for Combat - Look at Wasteland 3's system. Grid pops into existence at the start of combat, goes away after. Please tighten up the grid as well. At the moment, the 'area' for area of effects spells shows the underlying grid system and it shows why it makes combat feel so janky. They are so small that you even have tenth of a feet measured out. That shouldn't exist in 5e and it makes it extremely difficult to judge whether you might be in an AoE or not. The 'area' for AoE effects should be static. You can see great examples of this in Wasteland 3, PKM, etc.
2. Movement Indicator - Please give a better movement indicator. I often can't even see the 'line' because of all the surface effects or camera position. More importantly, I can't tell whether I'll be in range for a standard attack or not. Lastly, account for Difficulty Terrain. At the moment, when I start movement and it says I'll end up somewhere (with a yellow indicator for difficult terrain), but after I click I stop short because of the Difficult Terrain. Have it show the final movement location *before* confirming movement.
4. Rename "Backstab" to "Flanking" - Because that's what it is. Directional Facing doesn't exist in 5e, and it is clearly another port of a DoS effect. Make it so "Flanking" doesn't require you to be 'behind' your target, just on the roughly opposite side of them from another friendly character. With how janky the grid movement system is, it is impossible to tell whether you'll be in the 'backstab' position because you didn't port over the DoS backstab indicator. Flanking is an Optional Rule per the DMG, so use that.
5. Change Bonus Action Combat Tricks into Actions - Change all of the 'extra' combat options you added (Jump, Shove, Dip, etc.) to require you to use your Action, rather than Bonus Action. Having them be Bonus Actions right now severely diminishes the value of class features like a Rogue's Cunning Action, a Monk's Ki, spells like Misty Step, etc. Having a spellcaster Disengage from an enemy that has ran into melee with them should be a big sacrifice for the magic user, either in using their Action to Disengage, or by using a leveled spell.
6. More than 1 Follower - Allow for more than 1 'follower' per character. Right now, if a Ranger takes the Find Familiar Option, and then later selects Beast Master, they've just made their Find Familiar option completely useless because they can't use both at the same time. I've already had the game bug out and have both on screen so the engine can handle it. This will become more relevant as higher level spells become available like Conjure Animals, Animate Dead, etc.
7. Versatile Weapon Review - Add a button for switching between 1 and 2 hands for a Versatile weapon. For example, I might want my Ranger to wield his Longsword in 1 hand to take advantage of his Dueling Fighting Style. At the moment that's impossible without forcing him to equip a Shield. Tied to this is you need to check the rules for which weapons are Versatile (Spears, Quarterstaffs, etc.)
8. Spell Effects Polish and Removing Screen Shake - Please do another pass on your spell effects. Remove the screen 'shake' for when you select a spell but before you target it. Again, I know this particular effect is a DoS port and you like the effect, but it makes it really hard to argue that the game is an actual spiritual successor to the BG series. For example, https://www.youtube.com/watch?v=Zcu4KFZOKtM here are a multitude of spells being cast in the BG series. Each has their own feel to them (especially within different spell schools) based on the words used, colors, motions, etc. Right now, each spell in the game has a very 'samey' feel to them because of that obnoxious effect upon selection. It overrides everything else that's done to make spells feel different.
9. Add the Grapple Function - You clearly love forced movement (Shove as a BA and significantly stronger than 5e rules normally allow), so I'm surprised this wasn't already implemented. Grapple check to prevent movement until broken (or magic is used like Misty Step), and ideally then able to move the Grappled creature at half-speed (unless too big to grapple in the first place).
10. The British Are NOT Coming - Unholy hells did Larian only hire British voice actors and tell them not to try something, anything different? Seriously, this is just inexcusable. In the opening 1 minute of BG II we get about 4 different accents. There's no excuse why Elf, Human, Halfling, Goblin, Tiefling, and literally everyone else I've encountered in a game as diverse as D&D to *all* have British accents. Once you realize that, it just breaks immersion completely in dialogued cut scenes.
11. Redo Party Selection Mechanics - I'm sorry, but even in DoS and DoS 2 the party selection and movement mechanic was terrible. I can't count the number of times I had my party accidentally start combat or trigger traps because I forgot to 'unchain' them or whatever. The chaining system never worked well. Just go back to Shift + F1, F2, etc or highlighting in a box who you want to select on screen. It worked in BG 1, BG 2, IWD, Wasteland 3, PKM, and literally every other non-Larian RPG, and it did not work in DoS 1/2. Please change it.
12. Stylized Party Portraits - Come on, this was a huge deal in the BG series. The stylized portraits gave a lot of life to characters, even if their gear didn't necessarily match up. Right now the portraits are terrible and look like bland in-game models. There is so much of the UI ported from DoS, changing to stylized portraits would go a long way towards shifting the 'feel' of the game towards BG 3, rather than DoS 3.
13. Sanitize the DoS-only Ported Items - For example, Trap Kits. There is no such thing as a trap kit in D&D. You either use your Thieves' Tools or just a regular check. Same for Tongs, Hammers, etc. They are even the same in-game model right now, again shifting that needle towards looking like DoS 3 instead of BG 3.
14. Sanitize the DoS-only Ported UI Elements - For example, the Lockpicking progress bar. Have it be a rolled check, you already have that system in place. Change the hotbars to allow for selecting a spell level and seeing what's available, rather than having to slot every single spell based on spell level. Having 6+ Cure Wounds at higher level is going to be a PITA.
15. Remove 'Swiss Army Knife' Items to Enhance Character/Playthrough Uniqueness - Right now pretty much every utility spell has an 'infinite cast' item somewhere in the game. Speak with Animals, Speak with the Dead, Protection from Good/Evil, etc. This makes it so item hunting is more important than making meaningful character choices. Yes you *could* take that damaging spell....but then you might be missing out on being able to speak to that sus looking red cow over there. Those sort of meaningful choices are core to D&D.
16. Adjust Combat Ranges - Right now the game heavily, *heavily* favors Ranged combat. Fixing the free Disengage/Jump as a Bonus Action into requiring your Action would help this a lot, but just in general it is really hard to get into melee combat given how far away most fights start taking place, especially fights that are automatically triggered rather than by the enemy seeing you.
17. Fix Dual Wielding Thief Rogue - They can currently make 2 Bonus Action attacks. While I get that "Thief" rogues in 5e don't translate well to a cRPG, that is just broken.
18. Make Ladders/Climbing Cost Movement - Right now you can climb a ladder, vines, rock faces, etc. for free, without using any movement. This shouldn't be the case, as there are specific rules in 5e that make it cost double movement and even sometimes an Athletics/Acrobatics check. This also makes certain races/spells/items more valuable because they are capable of ignoring those restrictions.
19. Tone Down Enemy 'Special' Attacks - Sometimes a goblin is just a goblin. They aren't lugging around scrolls, 7 vials of alchemists fire, etc. Having every fight involve tons of mechanics from enemies just diminishes the value of when those 'special' attacks actually happen. Not every fight has to be special, sometimes it's just expending resources or for RP flavor.
20. WIZARDS SHOULDN'T BE ABLE TO LEARN ALL SPELLS - Sorry for the caps, but Wizards can only learn spells already on the Wizard's spell list. As much as I love having Bless and Healing Word on my Wizard, that isn't possible without Multiclassing or taking certain feats. I'm assuming this is actually a bug?
21. Only Certain Spellcasters Can Use Scrolls - Per the rules of 5e, most scrolls can only be read by a class that has the spell on their available spell list. They need to make a Spell Casting Ability Modifier Check (i.e. a straight Charisma for Sorcerers and Bards, Int for Wizards, Wisdom for Clerics) to cast spells from a scroll that are higher than they currently have access too (like a level 4 Wizard trying to use a scroll of Fireball to cast the spell). There are some specific scrolls that can be used by everybody (Scrolls of X Protection). This is another change to help the game feel like BG rather than DoS, because in the BG games that the way it worked, and it's how it works in 5e.
22. Ritual Casting - Self explanatory. Probably needs some balancing (longer ritual spells might trigger a random enemy encounter). But a Wizard shouldn't have to burn a spell to summon a familiar.
23. Identify Spell - Self explanatory. Identify an item was a key part of BG. Has a component cost (albeit not expended).
24. Spell Component Costs - Some spells are supposed to cost money to cast as a balancing feature, such as Find Familiar. Introduces a massive shift in the value of certain spells and how much you want to protect your familiar at early levels.
Agreed to almost everything.
Just a note on your point 16, I think that removing the jump/disengage will actually make it harder for mellee to get into fights. Even so, I am in favor of separating those two actions.
Another note on the same topic... How do you propose they adjust combat range? Because, unless most hostile enemies wait patiently for you to walk to their side, or you're always full on sneaking with a rogue, people will always call you out and get ready to fight from as far as they can communicate.