There are lots of solid ideas above.
The idea I would propose is “grouping”.
Currently if you pick up an item, say a silencer for a shotgun … its only useful for shotguns. Same with ammo and other shotgun accessories. It would be good to see in the inventory all the shotguns, ammo and accessories grouped into one display group. This would show you all the shotguns, the accessories and the ammo at the same time. You could see duplicates, make comparisons and check the stats within that group. It would be ideal if you could drop or tag for recycling in the same UI, and drag and drop the accessories onto the weapons. This would make one workspace for all the shotgun “stuff”.
Same idea for each of the weapon categories.
Placeable and throwable items could be treated the same, but without the need to manage accessories, I think its a bit pointless. Showing a group of all the throwable items together gives the player an overview of all their choices and the ability to compare weights and effects and make some choices.
Same with the placable items (gas tanks, landmines etc) showing all these grouped together would let the player make choices about the whole class of items they are carrying and get an idea of their options within a battle or ambush.
Clothing is a mess at the moment. There is a bug that when I currently go into a category of clothing, it seems to change my clothing to the default item selected in the category… even if I am just browsing. Then I have to remember to go and find the item I was wearing and re-select it before I exit otherwise its been reset. Not sure if this is a common issue or an edge case. Its just frustrating.
Anyway, I would really like to be able to drop clothing the same way I can drop other equipment. Same sort of UI, same drop mechanics. I would like a right click option where I could just click on the item, rather than using the keyboard all the time. Just as long as I can drop clothing to simplify my collection or give it to someone else.
The solution to many category management systems is tagging. Tagging for favories, tagging for recycling, tagging for type, tagging for rarity etc. If there are some automatic tags (weapon type, name, quality, weight etc) and some user applyable tags( favorite, to-be-recycled, … user defined or a set of colours or flags or something) then players can create their own system.
Then base your sorts and filters on the tags and you have one comprehensive system that players can use in all the different ways they like. Then on top of that, show stuff in groups.