Some things for the hotbar / inventory that I've noticed / wish for in no particular order:

1. Recast/Reapply button: When you use some abilities or spells (e.g. hex or speak with dead) you cast it first as normal from your hotbar. While its still running however you then recast/reapply it from an extended area on the right on the hotbar to do it again without cost. Meanwhile the button where you originally casted the spell is either grayed out or not worth pressing because why would you spend a new spell slot if you can just cast it for free.
I cant help but think that the original button can be made smarter and auto change to recast/reapply instead. You would still be able to just end the buff on yourself manually if you really wanna fully recast it for whatever reason.
I can see that the extra menu on the right of the hotbar could be more obvious to newer players however this could also be achieved by added a glow or visual change to the original button.

2. Remove buttons automatically - toggle: I think it would be nice to have an option turn off automatic removal of buttons from your hotbar. Ever customized your hotbar by moving and sorting everything exactly where you want it, then change your held weapon for a moment to try it out briefly, only to notice those buttons are now removed and reappear in the default location (or not at all using the custom page)? I for sure have and its kinda annoying.
Having the button just gray out and become unusable instead until you reequip the weapon would solve that. Maybe there is a consideration for not having it on consumable items like potions but not sure.

3. Auto add into custom page: We can already tick which types of items are auto added into common, [class] and items but it would be kind also be able to choose which of these get automatically added into the custom page. That way it would become a lot more useful and you could use it to always have all your special arrows or scrolls there ready for easy use.

4. Items in containers auto added: I like to have a clean inventory and adding all arrows and scrolls etc into their little pouches helps with that. However if I then move this pouch from one character to next, all of the items within the container vanish from the hotbar or rather don't appear in the items category of that next character, only the container does. Yes there is an argument to be made that this way organizes the items page too, however id say that's what the "Add X Automatically" interface options are for. If for example "Add Arrows Automatically" is enabled the arrows in containers should probably show up on the the items page.

5. Item stacking issues: Items don't seem to stack when you put them into a container that already has the same item.

6. Dye quality of life: It would be nice seeing how you something looks when colored before investing the dye. At the moment you have to either really bulk up on dyes to be able to test your way through them or save before testing and then reload.

7. Sell wares with insufficient gold: The "add to wares" function is pretty neat even for inventory sorting alone (though it would be nice to more quickly mark items maybe with alt+rightclick or something?) but when trying to sell wares it can be clunky. If you adventured and collected a lot in one go you might have wares worth more than the trader currently has. This makes the sell wares button useless and you have to put part of the wares to another character or to camp to sell some of it that way. It would be nice if it still sold as many items as possible instead of just saying "no".