Just an idea, Satchel.

My idea would be for a satchel, produced from leather surrounding a diamond. The leather for the satchel and the diamond  since this would be OP if you had cows. Lots of them. And wheat. But, if you right click this in your inventory, it would bring up a chest screen of half of one small chest. You can place items in here, so therefore this is more storage. Under Misc or Tools in the creatine menu. Can be crafted into a large satchel (1 small chest) by combining it with another satchel (Neither with anything in it.) You cannot place a satchel in a satchel, as that could constitute in infinite storage space. This would be effectively an inventory expander, and would be great in Minecraft.

-Its99