I had thoughts while soaping my hair (normal) and these thoughts were about RPG inventory (not normal. Are you okay?)
Broad assertion: in OSR games, one of the gameplay loops is about "managing inventory". Arnold and the GLOG drilled into my skull the usefulness of having Strength be inventory. Basically, your Strength score is equal to the number of inventory slots you can carry.
Now, it's important to note that this only refers to physical inventory. Things you can touch, etc.
Physical inventory isn't enough for me. I need more to represent The Things They Carried, which, as you'll recall, features a photo of Martha, Bibles, what it feels like when you don't want to murder humans (a cracking-popping-running sensation inside your chest), and how the stink of pig blood clots never really leaves. The title is about carrying trauma just as much as it is about literally humping 60 pounds of battle rattle outside Than Khe.
Now, obviously, half of those Things are real objects. The photo, the Bible. But they carry meaning beyond their brute physical presence. In game terms, these are personal objects, not crowbars, with no gold-for-XP value. But I'd argue they can be tools in a different sense: tools for a mental inventory.
Mental inventory is not a new concept. I think Red Kangaroo over at Library of Attnam first came up with it. Take all the things you have in your head, name them, and sort them. The higher your Intelligence, the more you can carry. Languages, skills, spells - toss em in the pile.
Space must be an issue for inventory to be effective. If things don't have to compete for inventory space, there will be no darling resource management loop.
I made a list, and even using skill ranks (oof) and then saying each rank ate up a mental inventory slot (bigger oof), most mental inventories are pretty sparse for first level characters. Say, about 6 things for wizards that speak 2 languages. How do I create pressure for space?
Well! Red Kangaroo also put together a pretty nice Darkest Dungeons inspired Trauma system for managing the stress of adventuring. This idea has existed around the fringes of the OSR for a while (I've seen Warhammer systems, Call of Cthulhu systems, GLOG systems, 5e Darkest Dungeon adaptations) but hasn't really caught on. Why?
Because inventory has to be able to flow between lists. It isn't enough for items to jostle for space on the list. A fascinating part of inventory is figuring out who can carry what, not just what can be carried.
Physical items can be transferred easily between owners. Skills? Languages? Spells? Stress?
Imagine four different cooks standing in front of four stoves. Each cook is given a random recipe, a time limit, and a random number of ingredients - some necessary, some not. They each have 1 square foot of counter space.
If each cook is able to move around, store things for the others, and assist them with the cooking process, the number of items won't be perfectly divided, but will reach an equilibrium dependent on what each cook needs, and the food will probably get made.
Now imagine the same scenario. Random ingredients, random recipe, time limit, same limited space. Except this time, the cooks are not allowed to help each other. Each one has to deal with their own items. Some will be fine. Some will drop ingredients or ruin their food.
These cooks are PC's. The first group dealing with physical inventory, and the second dealing with mental inventory.
Now, before we open the door to Vulcan mind-melding and hiveminds, some points must be made.
Physical tools are excellent because, in most cases, they are impersonal objects enabling one person to do a task as easily as another. Anyone can use a crowbar to kill headcrabs instead of opening things. This is part of what separates physical tools from mental tools.
Another part of that separation is investment. Learning languages, skills, and spells are all huge investments in D&D, and these mental tools separate some classes from others. If we allow transfer of this information, why have classes? How do we allow inventory flow without making character design irrelevant for characters that have disadvantages (doesn't know how to use weapons) to compensate for their advantages (can create illusions)?
A digression: For now, I think character classes are important. I'm not justifying my opinion here.
An important part of inventory is that the bad is just as easily transferable as the good. PC's should be juggling each other's Stress levels as well as making sure that everyone's boned up on their bombmaking skills.
Also, consider this: what's more interesting? Arguing with your party over how you're all going to spread out the stress of nearly dying, or silently calculating how many slots you can reliably fill with stress before going insane?
As a DM, I think player negotiations and problem solving around inventory space are engaging, if not always completely enjoyable for the players. But lasting satisfaction comes from solving difficult problems.
Also, how would PC's even transfer inventory to and from each other? What would they transfer? How long would it take? Can I transfer curses? What about class templates? What differentiates a skill from a class feature? What do you mean, that's just what they are? Is it okay to make a simulation of mental issues people actually struggle with? What kind of a DM are you?
Okay. The above are all valid questions. I'm not going to address them in this post. To back up my smack talk I need to write ANOTHER DAMN GLOG HACK to answer these questions with mechanics.
Well, guess what. It's written! It needs some final polishing, but, to fulfill a longstanding bet, I'm going to release it THIS FRIDAY.
TL,DR: The best kinds of inventory are the ones you can use to saddle other suckers with your shit.
And because I recognize the sweeping cop-out quality of this post, the Joesky Tax.
Photo of a loved one: As long as you possess this item, you may use it to store up to 3 slots of Stress. If you ever lose it, though, you gain Stress equal to twice the Stress in the photo.
Holy Book: You may take time once a day to pray with fellow party members. Whoever participates loses one slot of Stress.
The stench of blood: It follows you around. Every time you get close to someone new, they may gain Stress.