Items fall into several categories:
- Consumables(potions, etc)
The inspiration for this system comes from my books, where the characters have enchanted bags.
The inventory system should be robust and offer a lot of room. We don't want players to feel cramped but should punish digital hoarding to some degree. A quick idea would be to have "inventory" and "enchanted bags". The bags can hold practically anything, nested, and have no weight. The cost of this is that you can't readily access items in your enchanted bag.
Your inventory, meanwhile, is limited, but has the option for immediate use. So, anything you want to have on a hot bar, such as a tool, consumable, or use-item, has to be in your direct inventory.
We can start with a base of 20 slots, plus your equip-slots. In this, you have to fit all of your bags, and anything you want ready access to. Some kind of time-penalty should be imposed for opening a bag, and increasing penalties for deeper-nested ones.
Instead of just adding more backpack space, what about upgrades that act like equip-slots?
For example: a "potion bandolier" that provides 10 potion-only slots.
Or a "belt pouches" which provides a series of bag-only slots.
Your basic enchanted bag takes up 1 slot and contains 20 more slots. "specialty" bags should exist for specific item categories. These can be set to auto-gather so as you loot items they go immediately into that bag, but only if it's in your inventory and not inside another bag.
Larger bags can be available at a cost. Ultimately, a character could have their entire collection of everything on them at all times, provided they had enough bags.
Materials and Consumables should stack. Everything else should not. If we go with a basic slot-system, then every item or stack takes up 1 slot.