Add a category for items that have no gameplay value ("junk" category), so you know you can sell it or trow it away.
Store the books content in a journal somewhere, so you will have access to them anytime without carring them around. Or move it to "junk" once you opened them.
Keys should be automatically removed once you opened the door or a chest it was made for. Or again - move them to junk.
Subcategories in categories. In crafting i would love to have tools (like inkpot and quill) and ingridients separated.
I think it was Witcher 3 that marked books as already being read, both in your own inventory and in vendors'. That'd be quite nice, as I can never remember which ones I've read. I also agree that keeping the books in a journal would be nice: I'm not always in the mood to read them if I've just collected them in a combat-prone environment but I'm likely to be in a more receptive mood in a settlement, but by that time I just want to empty all the junk from my inventory.
And, yeah, some means of subdividing the ingredients category would be nice as it quickly becomes a fairly unpleasant task trying to find things. I still wish there was a way of switching between the standard Morrowind-like interface with icons and pop-ups and the Oblivion-type list with short descriptions: both have their uses and both can be frustrating, so having the choice would be nice!
Edit: apologies if there's some repetition, four hours after crawling out of bed, I'm still not properly awake yet...
