The concept is simple, you walk up to another person and with nothing in your hand you hold right click on that person. It opens the other persons inventory as a it would a chest.
Now before you go saying it's overpowered, I will extrapolate on some particulars.
Firstly, you wouldn't be able to access their inventory instantly, it'd take maybe ten second of holding right click on them to 'pickpocket' them and then you select what you want.That would keep people form just running up to someone and grabbing their stuff and running away, it would require stealth.
Secondly, you'd have to be withing three blocks of the person to attempt to pickpocket someone.This would mean you'd have to get close to the other person to even attempt to pickpocket them.
Thirdly, if the person moved out of reach while you were trying to pickpocket them or while you were pickpocketing them, it would instantly stop and you'd have to try again, although any items gained before they moved would be kept.This would make pickpocketing an art, a skill to be practiced and perfected.
Lastly, if you are hit by anything while attempting to pickpocket, you will take double damage form that attack and the attempt will end.This would make it dangerous to do, keeping everybody from trying it for fun, because even if the person you're pickpocketing doesn't notice, if one of his friends notices and shoots you, you're as good as dead.
In my humble opinion it'd add a whole new aspect to the game that would make the game a bit more interactive on the multilayer end, making more options to gameplay and it'd add the ability to have many more self imposed goals like "steal nine diamonds from miners" or "steal the whole inventory of a clan leader". This would make it so there could be 'thieves guilds' or some such things.
Pxex: "when you get pickpocket a message appear only to you that say something like "You feel lighter" or something that will alert the player, so you cant just steal there whole inventory while there mining or crafting "
Jorriem: Agreed, a text that appears would be nice, but it'd have to be subtle so only if the person being pickpocketed was paying attention could they stop the thief.
Natemma: "Couldn't just standing behind someone be mistaken for pickpocketing? It would definetely cause some unprovoked attacks. There should be a way the person stands or acts when he is doing it, so it is unmistakable."
Jorriem: Agreed, there should be a unique animation for someone pickpocketing to make it more obvious to observation.
Now that's my general idea, it could use some embellishments and improvements, but I think it's pretty good, comment and give me your suggestions and critiques.
(A couple notes, it could be difficulty based, so only on hard could you pickpocket, or it could be an option in the multilayer properties so servers that don't want it could turn it off. Also it could be affected by sneak, i.e. if you're in sneak it take less time to get into someones inventory or you could only pickpocket in sneak)