My idea is doors that have a passcode and keys to lock and unlock them. This idea isn't coming out of nowhere, it is coming out of a problem that I have somewhat often.
You see when I play multiplayer I have "friends of friends" on. Most of the time everyone respects other people's builds and stuff. However, some people are just rude. Once in one of my worlds I had found 8 diamonds from chests in the blacksmith shop of a village and the nether fortress. I also had a bunch of other valuable things in my chests. I went off to help a friend build a trap and left my house with all doors closed. When I returned to my house, I found a friend's friend had taken the ENCHANTED armor I set aside for myself, my diamonds, my emeralds, my saddles, my diamond and gold horse armor, and my emeralds. I just managed to kill him and get my stuff back before he left. Now what gets me mad is that I had labeled the doors to my storage room saying it was my private storage and not to steal or enter without my permission, but he entered anyways. That is why I want keys or passcodes for doors so I don't almost lose all my valuable stuff.
GIve your thoughts in the comments and answer my poll.
Well, you could get creative and build a solid home defense system. Or make traps to try and fool unwelcome guests. Lockable doors just seem a bit... boring. And if you really wanted to, you could make a locking iron door using some Redstone.