7 Alternate Versions of a Bag of Holding

1. Spell: Gabristin's Dimensional Chest

Gabristin's Dimensional Chest is a middle-ranking spell that creates a dimensional storage space that can be summoned or banished as needed. When the spell is cast it must be anchored to a physical object, typically a key. After the first casting, the chest will remain as an active spell and remove access to the appropriate spell level used to cast it. The dimensional chest can be banished by the caster, freeing the spell level for other use, but if so, anything in the chest at the time is lost to the ether. If the chest is hit with a destroy magic spell, exposed to an anti-magic effect, or forcefully dispelled, everything in it is lost to the ether. The chest cannot be targeted by spell effects unless it is physically present, so a magus can have a dimensional chest, walk through a dead magic area, and still have it on the other side. While in the area, they would not be able to summon it. The summoning causes a chest of the caster's design to emerge from the earth on a suitable pillar or plinth. They can then access the storage space, do what they want, and then close it, sending it back to the elsewhere. While it displaces things on its appearance, this damage vanishes as soon as the chest recedes. 

The Sorceress Gabristin pioneered this spell allowing for the storage of 20 items, regardless of size or shape, so long as they can fit through the top of the chest. As an accomplished sorceress, she kept a variety of magical garments in the chest, several figures of wondrous power, and things mortal were not meant to know. Unnamed variants of the spell hold fewer items, have more restrictions, and are more vulnerable to countering. When she created the ultimate version of this spell, her name was made part of the fundamental casting.

2. The Peculiar Pet of Amanda Pekkarenin

Amanda Pekkarenin was a summoner of no small skill, and she was attended by the strangest entourage of the royal courts of Palisander. Her retainers were animated pieces of furniture, mimics, and golems. One of her most prized companions was a mimic the size of a steamer trunk that doubled as a living inventory system. The capacity of Pekkarenin's mimic was unknown, but was assumed to be impressive as on its demise it disgorged its contents, including several swallowed head of livestock, over a dozen humanoid corpses, a ton of inorganic material, a small fortune of gold and gems, a large fortune of magical items, and the missing corpse of Amanda Pekkarenin, deceased of old age before the mimic consumed her and began a decade long serial killer career before being slain by a trio of dragoneers.

The Spell to Summon a Mimic of Holding remains a viable spell, though it is inadvisable, as it is still a mimic and as such is a voracious ambush predator.

3. The Butler's Butlery

A signature spell of the Dungeon Butler occupation, this creates a soul-bound magic effect that gives the caster access to a permanent dimensional storage space that only they can access or be aware of. As they increase in level and power, the capacity of their Butlery also increases, where a first level Dungeon Butler might be able to stow a backpack worth of goods in a secret dimensional space, the Master Butler might have the ability to store hundreds of items with no concern on weight and minimal worry about size. 

The Sneering Servant is a tavern in Terrasquestone that caters exclusively to Dungeon Butlers, and to become a member of this organization, an applicant must have the ability to carry enough provisions and supplies to non-magically set a proper dinner table for twenty guests. There are several dozen members of the Sneering Servant Tavern company.

4. Ozlum's Organizational Orrery

This spell was created by a dragon who took great interest in spellcraft, polymorphing, and collecting. The spell reduces physical objects to energy patterns that are stored in the aura of the caster. When accessing the spell, all of the different items in their orrery appear as ghostly outlines orbiting them slowly in sorted rings, and reaching for a single item is relatively easy. The spell is most popular among alchemists, potion-makers, and assassins who use it to maintain their inventories of ingredients. The Weapon Masters of Nulgrosh use this spell to carry arsenals of weapons into cities while looking unarmed, and suddenly manifesting piles of weapons and telekinetic abilities to create dancing weapons and walls of blades.

The Orrery is an effective spell, but it not very popular because when accessing the inventory system, the caster can really only see the glowing rings and equipment, meaning it is horribly distracting in combat, and generally only useful in crafting situations.

5. Merchant's Manifesto

This NPC class spell allows merchants to carry large inventories of goods to sell as well as currency. It is similar to the Butler's Butlery, but if the merchant is slain, their inventory vanishes and is returned to the Merchant's Guild Center where they were registered. This prevents merchants from being slaughtered to steal their merchandise, as well as keeping said investment in equipment from being swallowed by the ether. The limits and capacity of this spell are unknown, and the merchants only have access to what they are carrying to sell it, and thus a weapons merchant cannot wander off and become an illicit bandit leader with a full arsenal. They can become employed by bandits, raiders, gangs, and tribes, where they become the organization's quartermaster, forcing said groups to buy and sell gear through the merchant, enriching the Merchant's guild.

The Merchant's Manifesto is a spell that allows a magic-using merchant to functionally have a soul bound scroll that acts as a portable inventory system that they themselves have limited access to, and that is tied to a central warehousing system.

6. Vardivia's Voluminous Volitus

It is fairly common for small storage spells to be placed on pieces of jewelry such as necklaces and rings, with the general caveat that said storage devices tend to have limited capability, only holding a cubic foot worth of gear, 2-5 small items. The Archmage Vardivia overcame this limitation by swapping out a typical magic resonate crystal with a Beast Core. Her most famous Volitus was crafted from a large core harvested from a seven-headed hydra that was dozens of centuries old. She wore the Volitus concealed as part of an overly ornate and tacky belt, and with it had a personal inventory system large enough to hold the contents of an entire manor, or country household, furniture included.

The basic flaw of a volitus is that they can be stolen, or with long enough exposure, magically breached. They also have a slight habit of exploding when they encounter an anti-magic effect, turning their contents into shrapnel. 

7. I don't know

I didn't plan this far ahead and am honestly astonished I made this dumb idea stretch out to 6, honestly.