Following a discussion on the OSR Discord... I made this in 15 minutes. You've been warned.
BAG OF HOLDING
You are an intelligent bag of holding.
Starting equipment: 1d4 randomly rolled items from your last owner. 1d100 copper pieces.
Your armor is as leather. You have no hands nor legs, but can manipulate things with your mouth/cover and can slowly shuffle around (movement speed 10 feet/round). You can see and speak. You hold 6 slots per template, and take one slot to carry.
A: Hold item hostage, Appear as a bag, bite
B: Secret Stash, detect dimensionality, Spit out items
C: Tougher bag, Faster bag, Hold someone
D: The great devourer, Astral Plane excretion
Hold Item Hostage: You have the power not to surrender your items. You have reasonable demands - a ration a day in food, you won't store gross stuff, no unsheathed weapons etc.
Appear as a bag: You can easily appear as a normal inanimate bag.
Bite: You can bite for 1d6 dmg
Secret Stash: On top of the 6 slots per template, you now have a secret stash of 5 slots for your own personal use. 1/5 of these slots can be for an "unspecified item" (just like the thief).
Detect Dimensionality: Putting extradimensional spaces inside other spaces is a bad idea. You can detect such areas (or the openings to such areas) within a 100 feet.
Spit out items: You can spit out items with great force. If suitably sharp or hard, the item does 1d6 damage. Range: 20 feet, -1 penalty per 10 feet beyond.
Tougher bag: Your AC is now as chain
Faster bag: your movement is now 20 feet/level
Hold someone: A single human size creature can carefully climb inside you to hide. This take 15 slots of space. You can carefully renew the air supply. To the person inside, it feels like hiding inside a moist sleeping bag.
The great devourer. If you roll max damage when biting, the target must save or be devoured whole and be expelled into the astral plane. The target cannot be bigger than an ogre.
Astral Plane excretion: You can get rid of things permanently (?) by excreting them into the astral plane.
1: your ex owner was a burglar. +2 to stealth
2: Your ex owner was a wizard. +2 to saves vs magic
3: Your ex owner was a farmer. You have a turnip familiar.
P.S. I wonder about the ecology of bag of holdings. Perhaps when a bag of holding "retires", it gives birth to a new, smaller bag of holding? It has "genetic" memories from its parents, so even though it might be days old it has the "life experience" of an adult.