Backpack *Enchantment*

Not a new craftable item,
but an Enchantment,
exclusive to armor,
with up to 3 levels,
each level adding 3 inventory slots
off to the side of the main inventory,
for a maximum of 9 extra slots.

Not a replacement for shulker boxes.
Not functioning like shulker boxes,
(which must be put down in the world to be opened)
ALSO not functioning like shulker boxes is that this enchantment
DOES NOT
FUNCTION AS
A CHEST
You cannot place it on the ground at all and expect it to hold items.

If you take off the chestplate enchanted with Backpack, all its items fall out.
This is a balancing mechanic to make it not replace shulkers

Also of important note:
A player acquires access to shulkers after defeating the Ender Dragon.
Which is also when they acquire Elytra.

Once a player begins wearing Elytra, which CANNOT be enchanted with Backpack,
they begin relying on Shulker Boxes instead.

It is crucial to progression and balance that a player must choose whether they want to fly, or whether they want inventory storage slots.

It's already the 'meta' now that a player stops wearing armor and switches entirely to wearing their elytra at all times once once they gain access to elytra. This, however, will provide incentive to still use armor.

Further balancing: The Backpack-enchanted armor does not automatically pick up items from the world. When your inventory is full, you must open your inventory and then move items over to the backpack grid yourself. This is to keep the player mindful of what they have placed in there.