One of the major problems for vanilla multiplayer right now is that there is no way for players to secure their items easily.
It would be cool to have a lock item which could be placed onto other objects in order to lock them. If anyone has played rust it has a pretty decent system of locking, players can create a lock and then create a key from that lock which they can use to open the item.
The idea of locks opens up a lot of possibilities. Lockpicking could be implemented giving players a chance to break into locked objects. Locks could be upgraded using metals to improve their safety. You could go even further and have the metal chests provide more protection so people could store their valuables in secure iron chests that couldn't easily be chopped into. Something like lockpicking could be given as an option that the server owned could enable or disable so peaceful PVE servers aren't full of raiding.
Different ways of storing could have different locking methods, a key might be fine for a door or a chest but maybe a crate needs a metal chain to secure it properly?
This idea would allow vanilla servers to function without players being able to just walk in and take stuff or venture into peoples property.
A Minecraft mod called Civcraft implemented a feature that allowed players to 'upgrade' blocks using materials. You could for example add iron to blocks to make it take longer to break. An object that would allow players to upgrade placed blocks and items would be a great way for players to secure their items, it would allow for metal bunkers and vaults where players can secure their most valuable loot, things like banks would become something that wealthy players could invest in and allow access to. I picture something like an upgrade hammer (some kind of modern looking hammer) which you could use to add materials to an object. Adding more material (iron for example) would increase the amounts of hits it takes to remove the block.
Players could use this feature to upgrade their doors and walls of their homes to prevent greifing, along with the locking feature this would give players some security in their properties while they are offline, any greifers would have a harder time.
Hopefully I didn't explain this too badly