Ender ball

They could add ender balls.
How it would work: They would synchronize all the containers in which they are placed. This means that if we place one in a chest, another in a sulker box, and yet another in our ender chest, all three will have identical inventories. This means that if we put a dirt block in our ender chest, it would appear in the normal chest and sulker box, and if our buddy removes this block from the normal chest, then it will also disappear from the other containers. Each player can craft unique ender balls, which no other player can craft. He can, however, give them to other players. Ender balls from the same player are linked by their names . If they are placed in a container which already has other items, then as many items as possible will be synchronized, the rest will be expelled from the container. If a player has an ender ball with his name in his inventory, then it will synchronize the player's inventory with other containers. Synchronized containers will have the size of the smallest synchronized container (or the largest, but this is likely to be heavily exploited). Ender balls cannot leave the container in which they are placed (e.g. via a hopper) unless a player picks them up. However, other objects can be funneled out. Two players can have their inventories synchronized if each of them has a ender ball they've crafted that bears their name, and a ender ball the other player has crafted that also bears their name.