Summarized suggestions 1. Spellcasting UI by spell level separate from the hotbar. 2. Spells/abilities that can only target self or have no target should activate in one click. 3. Shared party inventory/bartering. 4. Left alt toggle option.
Spellcasting UI I saw a few people mention this but it needs to be repeated. Spells currently take up too much space in your hotbar. By level 3 I already have more spells/abilities/useful items on my hotbar where I have to scroll to a second page. This problem will get way worse as you continue to level up.
I think a much more elegant solution would be to have your available spells (prepared spells for casters such as clerics, druids, wizards etc.)/(known spells for casters such as bards, warlocks, etc.) show up when you click the spell slot rectangles above the movement bar. So if you wanted to cast sleep, you'd click Spell Slot I, then click sleep, then target and cast it. For spells upcast at a higher level, for example casting sleep with a level 2 slot, sleep should show under Spell Slot II, perhaps in a separated position above your normal level 2 spells. If you adopted this method a cantrip rectangle should be added as well.
I think this would really clean up the interface and if players wanted to they could still hotkey spells they use very regularly and have the rest of them organized just a few clicks away. A less elegant solution would be to allow you to cast spells from the spell UI (default hotkey K) but it would be pretty unintuitive since it shows all your character's spells at once.
Self Target/No Target Spells/Abilities Spells and abilities which can only target yourself, for example Mage Armor or Dash, should not require you to click the button and then click yourself/a random spot on the screen. I find this very unintuitive and inconsistent as abilities like hide happen immediately. I've watched streams of new players who struggle using Dash because they click it and nothing seems to happen.
Shared Party Inventory Micromanaging weight and bartering was extremely frustrating in DOS1&2 and remains frustrating in BG3. Please just have a shared inventory that uses the combined carry weight of your party.
Left Alt Toggle Let players enable an option to toggle left alt on or off instead of having to hold it down 90% of the time. Once again I found this very frustrating in DOS2 and would love if this option was made available in BG3. I understand not everyone wants to see all the tool tips all the time but for those of us who do a toggle would be greatly appreciated. Also I agree with the several other suggestions that anything clickable (and not hidden) should show up when left alt is pressed down/toggled on. In addition, an option to hide empty containers/bodies (instead of marking them as empty) would be very welcome as well.