I would add a thief they would be a mob that would take an item from inventory or chest and then run. the only way to get your item back was to kill it other wise your item is gone forever. it would take valuable stuff like diamond gold and iron also if you killed it might also drop valuable.

It would have 20 hearts also hostile mobs would target them and if killed by a zombie they have a small chance of become zombie thief these would be easier to kill and would stay and fight and deal 3 hearts.

There would also be a thing called a vault. the way to create a vault is to put a chest in the middle of a of a ring of iron ingots and this cant be looted by thieves. the vault is also a place that the thief store thier loot. then you can then use the items in the chest if you have a key which is used to open vault an created by putting two gold nuggets in any formation.