I have two ideas, but you need to make mob heads stack-able first before implementing them. Players can only use the quick-access bar. The rest of your inventory is filled with barrier blocks. This sets up a cool mechanism with backpacks. A backpack in your quick-access bar removes some of the barrier blocks. I'm connecting two ideas. The slots removed of barrier blocks can now be used like the quick-access bar. It's the inventory of the backpack in your quick-access bar. Bigger backpacks removes more of the barrier packs. You can only have one backpack in your quick-access bar. If you have items in your backpack inventory and you no longer have the backpack in your quick-access bar, all the items are dropped. If you swap the backpack in your quick-access bar with a "deflated" backpack in that backpack inventory, the items that can't fit into this new backpack are dropped. There can never be a backpack that exceeds the size of a player's inventory. The best part about these backpacks is that it handles ender chests and shulker boxes well. You can look at those items as being magical in nature. Mojang already programmed shulker boxes to not allow shulker boxes inside them. The biggest problem with these backpack is when you want to save items into the backpacks' inventory. The solution to this is simple, but you will have to store the id of every backpack and the items in each backpack into some database. In other words, this is more memory that you need to pay for to have on your server. I'm going to stop here because the longer your comment on Planet Minecraft is, the slower it is to submit or edit it.