I know everyone's probably thought of this before. But wouldn't it be nice for that wee bit of extra space? Being able to build a backpack and equipping in a space like 'the shield'? Perhaps you can make better backpacks that expand your inventory with more space?
Did you forget about the Shulker Box? I know it's late game, but isn't it a bit of a luxury? Inventory management is part of the game, and Shulker Boxes should be when you've seen everything there is to see and you're ready for some MASSIVE build projects. (In my opinion)
Perhap's OP too scared to go into the End looking for Endcities & want's an easier way to carry more than he needs.
As for a 'mid stage backpack - why not have an enderchest & for an 'early stage' - find llhama/donkeys & have a pack animal