Not sure if it's been proposed yet, but chest looks would be awesome. How'd it'd work, is first you'd craft the lock and keys. Upon placing the lock on a chest, the lock disappears, and in it's place appears 4 keys. Those 4 keys only work with that lock, and you can distribute those keys to whoever you want as well.
Suitable formula/recipe for the above idea.
Another way, would that you craft locks separately from keys, and only YOU can open your own locks. That would be a bit less fun for people taking up thieving in SMP, but meh. Even if you kill them and take their keys, in the end, after killing 50 people, you'll have a bunch of keys and you won't be able to tell which key opens which chest.
Other designs could work too, but I'm just making gold a bit more useful.
LOCKPICKS NOW, they would be a bit more tricky to handle. Imagine the tumbler minigame while lockpicking in Oblivion, except with only 3 tumblers (because hard and very hard chests pissed me off). You could craft a lockpick with this...
Lockpicks would only have one grade/material, and each time that formula is used, it gives you 4 lockpicks, each only working once.
And if the tumbler minigame isn't implemented, a simple 1/10 chance would open the lock.