My take on a "back" inventory slot and Backpacks.

This is a simple concept, and I believe it has been partially done in mods. I suggest that there should be a separate inventory slot for the "back", separate from the chest. This is where Elytra and possibly a backpack would go.

A Backpack, when equipped, can accommodate for another 27 inventory slots, including Shulker Boxes. It would not, however, be able to store items when it is not equipped, as that would effectively be a Shulker Box 2.0.

As many have said, as the number of unique blocks and items has increased, the inventory space (excluding Shulker Boxes) has changed by a grand total of one slot (offhand) since version 1.0 if I am correct. Two key parts of the current survival meta are inventory space and Elytra flight. Backpacks would still function without a back slot: they could just take the space of a chestplate. A back inventory slot and Backpacks could allow for a better gaming experience and improve the current state of the Minecraft inventory. It would let player choose between flight and extra storage, each possibly compromising protection should there be no back slot.