If you want to improve the design make the top category icons smaler and change or remove the border from them they look too bulky - Instead it coud show one more item. I currently have 136 inventory slots so It coud show more items on the left row, item icon coud be smaller and border around the name too.
You may also want to add some sub-categorys like sort by weapon type.
Here is an example from The Elder Scrolls Online, there is also an option to change quick between onebag roster layout and the diablo style:
