Add small charge borders around charges and bag number in backpack UI. Texture path: ui\widgets\console\human\commandbutton\human-button-lvls-overlay.blp
Group slots by bags of 6 slots. The grouping might require frames and smaller icons for the inventory slots. Maybe just use the numbers in the top left.
Change background texture to empty inventory texture.
Show the item icon bigger in the tooltip frame.
Add sell buttons #1348 and #1349 which would require the player to select a specific slot.
Maybe add a swap items button and allow selecting two slots.