Thanks for compiling this great list! I'm going to try and avoid suggestions I've already seen, though I'll give a quick +1 to a lot of things, including especially party size (please pleas please Larian, change your mind about "committing" to just a few companions -- your own listed inspirational games both used 6-person parties. It won't break the game. It will be extra work.. but you're already doing that anyway. LET ME TAKE ALL MY FRIENDS).
Anyhoot, on to actual suggestions!
Please show more information on the character sheet panels. There are drop-down menus already, but even they don't contain much in the way of information. For example, I couldn't see the warlock invocations I'd taken, even though I knew I'd selected them (partially I wanted to make sure I had selected them, as Devil's Sight seems to not work on Wyll). Having more information about choices that are coming up would be good too. A preview of what's to come at later levels. I have the benefit of a lot of experience with D&D and fifth edition in particular. Many of your players don't have that and it can be tough to feel like you're really planning your character when you don't know what they'll be able to do. The increased information isn't going to bother those who won't care to see it, but it will be helpful for those who want to be more deliberate about plotting their characters' progressions without having to either buy a PHB or pull up D&D Beyond at the same time and pour over things.
If I fail a passive Perception check, don't tell me about it. This alerts me to the fact that there's something here I'm not seeing. A DM with players can (and should) keep track of their players' passive senses to compare them to traps and hidden foes. Watching my whole party whiff a Perception roll and then walk into a tripwire just makes me feel like a rube. Don't make me feel like a rube. And, then when I do notice something, I feel even cooler about it. Hurray! My ears/eyeballs work! Or Shadowheart's do or something.
I know Sven has mentioned the incoming (sometime) option to change the classes of NPCs. I am very much against this, though especially if Larian sticks to its guns about the locked party size being 4, it will be pretty necessary. Even if that isn't the case, I would desperately like to restat my NPCs from the ground up. Honestly, I'd like the ability to change some of their racial things too. Not change the race itself, but I mean Astarion having minor image as a cantrip makes sense thematically, but the spell isn't very useful and I'd rather give him some other cantrip. I also wish that variant human were a thing because both Wyll and Gale would benefit far more from that. And, I mean, you're using variant rules already (two subraces of drow? potions as a bonus action? everyone apparently has cunning action?), so surely this wouldn't be too tough to implement?
More tooltips and tutorials that you don't have to hunt down. I didn't know how to just sell something for gold. The button to do it looks like a scale above the "Barter" button, but there's no tooltip explaining what it does, and I at least didn't automatically think "oh that means gold for the item I put in the barter box." The hotbar is also a hot mess, but I think enough folks have probably remarked on this that it's a known issue. I like the idea of quick items (potions, scrolls, etc.) being its own "tab," then having separate tabs for spell slots. Click on my second level spell slots tab and I'll see all my second level spells I've prepared, as well as my first level ones cast at a second level slot. A rundown on spell slots would probably also be good for folks who have no idea what those are. I only knew to go looking for how to upcast magic missile or healing word because I've played D&D before. Folks who haven't aren't going to realize that's even a thing without you explaining it to them. I'm sure it's in the tutorials menu, but I shouldn't have to go to a separate menu to find out how the main resource of a class works. Your game should tell me when I first come across it. Give veterans the option to turn the tutorial pop-ups off.
Please give us an inventory autosort. For the love of God, Montresor.
Breaking my promise to not mention things others have brought up again to specifically call out sheathed weapons. I don't think I've ever seen anyone carry a rapier on their back. Except, perhaps, in Dragon Age and it looked just as ridiculous in that game too. It's even more absurd when it's, say, a dagger. If you're going to show us idle weapons on our person, do the same for shields, please. I'd honestly prefer no idle weapons showing to having every single weapon stored on the characters' backs.
I'm not sure this isn't in the works, but more voices for player-made characters would be nice. I would like more variety than "British" or "British but a little lower register."