CONTAINERS! I really want to manage my inventory.
Ammo Belt - For storing arrows and throwables, obviously. Should work the same as the containers we have now.
Scroll Case - Same, but for scrolls
Potion Case - Same, but for potions and poisons.
Wares Crate - That automatically stores stuff that is 'added to wares'
Technically, you can do this now. It simply misses the functionality of having any newly picked up items going into the containers even if there's a stack of that item in there... You have to keep manually organizing everything you pick up.
I typically only bother with unique items these days because of it. Putting my Shovel and Pixie Bell into a sack so they're not in my shop UI and can't be accidentally sold. Same with any magical items I want to have on hand (Like Periapt of Wound Closure, Mystra's Grace, Smuggler's Ring) put them in a sack and they're out of the way in the shop UI.
Bags of Holding - that decreases or negates the weight in the bag. Make it expensive, it;'s fine, it's for us hoarders anyway.
Technically, we did used to have something like that. But it got nerfed because it allowed everyone to run around with 100 barrels of explosives.
The Chest of the Mundane in the Arcane Tower used to only weight 20kg and since anything put inside it transforms into random trinkets weighing between 0.01kg and 0.5kg this made picking up a million 60kg barrels was no problem even for 8 Str characters (I used to get around the slight confusion aspect of not knowing exactly what items were by organizing it with Crates. Crate 1 had Firewine barrels, Crate 2 had Oil barrels, Crate 3 had Smokepowder barrels etc)
The chest now weighs 100kg so you need 24 Str to pick it up and it will take up a considerable amount of a characters carryweight on its own. But it still allows a high strength character to become a mule that carries TONS of heavy stuff shrunk down into tiny trinket sized form (Of course, manual organization is even more important so you can actually find specific items inside it)