When moving items in around the inventory it would be nice if they swapped places rather than just refusing to move. Because items are different shapes there is a mini-tetris game of inventory management stacking stuff properly. If I have picked an item up (item A) and I drop it on another item in the inventory (item B) it should be placed in item B’s “space” and item B should become the item I’m moving around.
I’d extend this to the items on the ground. So rather than just getting a “you have no space” message when you try to pick it up you should be able to pick it up and it’s in that same “moving item mode” where you can choose to put it in your inventory, replacing another item.