Backpacks add an amazing dynamic to the game, allowing one to store extra items in a visible backpack and even keep items on death! However, there is an aspect that seems a tad overpowered; one can make as many backpacks as they want and carry around a lot of additional inventories, effectively turning them into early-game shulker boxes.
Perhaps it would make more sense for balance, given the persistent nature of backpacks, to have them all share a single inventory, similar to ender chests. That way any backpack you open would have the same contents.

It's sort of too late to implement something like that, since it would break all existing backpacks. It's a good suggestion though