The Mystery chest seems pointless. If you are able to get to the middle and destroy it you probably already have decent armor and weapons.
Some new items that I think could work well in the mystery chest:
-A multi use ender pearl
-A player tracker that gives other players the glowing effect when you hold it, that way you dont have to search for them if they are hiding.
-Enchantment books that you can put onto your armor. The enchantment books would be for a specific piece of armor, all you would have to do is right click on the enchantment book and it would be applied. (For example one of the enchantment books could be called ‘Thorns For Chestplate’ and when you right click your armor will be automatically enchanted).
-An enchanted golden apple
Thank you for reading! Have a nice day.