So I was thinking about a few things and I came up with the idea of adding a left hand slot in the inventory. Your normally selected 9 items go into your right hand but the left would be good for including other things.
Bow and arrow: Equipping arrows would essentially act as your quiver. This would give you a limited supply of arrows as well as let you choose what types of arrows you use in the future, (wood, stone and gold in the case of iron being in short supply and the addition of fire arrows)
Shield: carry a shield in your left hand to aid in combat. Right click to block with a shield equipped
Torch: Carry a light torch with you at night/underground so you can see to move around without having to leave torches everywhere.
You need to be able to craft a bowl of milk and place it somewhere then the cats will stop following you. I had a few but it was impossible to tame more because the cats following me would spook the ocelot .
It's definitely not a left hand holding sand. The block you are holding also is hidden when the hud is, so you cannot see the block and not the hud. The picture doesn't look like it has had dimensions altered to erase the hud either.
The left of the chest also doesn't look like it has a block sitting next to it or at the corner because you can still see the side of it.
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Bow and arrow: Equipping arrows would essentially act as your quiver. This would give you a limited supply of arrows as well as let you choose what types of arrows you use in the future, (wood, stone and gold in the case of iron being in short supply and the addition of fire arrows)
Shield: carry a shield in your left hand to aid in combat. Right click to block with a shield equipped
Torch: Carry a light torch with you at night/underground so you can see to move around without having to leave torches everywhere.
Possible many many other uses for a free hand.