Hands should have two modes that will change automatic depending on stuff that you have put in hands: Equip mode and Storage mode.
Every hand will have 3x3 storage.
Equip mode will be activated if only one item is in hand slot.
If this mode activated you can:
Interact with items
Fight using them (If melee/gun equiped)
Some of items that forced to be two-handed (Like sledgehammer or HMG) will change mode to storage mode if put in one hand. To put item in two hands you have to place it beetween Left hand and right hand slot (If you know how to make this better - write in the comments). Items will automaticly equiped in both hands if hand slots are clear.
Storage mode will be activated if more than one item are placed in hand.
You can’t use items if this mode activated.
To increase amount of space in hands you have to equip bag. But if bag is equiped mode will change to storage mode permanently untill you unequip bag.
So umm, this is all my suggestion.
Fell free to add your own suggestions in the comments.