I didn't know how to describe these ideas in the topic.
First, I'd like to suggest a "non lethal" dueling method, where the loser is just knocked unconcious. I'm sure I've seen this method of pk
suggested here before, so I'm just casting in my vote.

But in addition, here's my idea for a "lethal" pk system.
One of the major concerns with pk is griefing, and a yes/no system for "pure" (lethal) pk is kind of combersome.
The devs also don't want people fighting eachother in the streets, is just wouldn't look right in the major "protected" cities.
(one day I might like to discuss/argue just how much control players can have in the laws of the land, but that's another topic)

So, you either limit pk, or just ignore the whole process because you have better things to do.
But, like it or not, a lot of players enjoy pk, even those who don't participate in it themselves. I'm one of those strange people who likes the feel of danger, but will only attack in self defence (and find nice ways to run away). So, I'd like the option to "turn on" that danger, whether or not I want to be a predator.

So, back to the point. Griefers, and atmosphere.
Let me make a small remark on atmosphere. I hate good and evil. I hate people vs monsters. I love moral ambiguity, anti heroes, and
regular jerks who aren't "bad guys" but just regular people.
I can accept the races living in peace as a whole, but there's got to be the shady characters in the shadows.
I don't mean mobs, I mean players.
Don't get me wrong, I can play evil without blood, like I said I'm not a Twitch Killer. What I'm getting at is that players should be held
accountable for their evil actions, prison shouldn't be the end all answer to these problems, some people should be able to fight back.
Now, I'll end the rant and get to my point.

How would a character get involved in these shady processes? How do you earn an "unsavory" and illegal proffesion?
Simple, you do quests.

I propose a certain "notoriety" system. You get some kind of arbitrary rank that reflects your legal status.
To raise noteriety you complete quests, or even kill a person.
Here's where the trick comes in, you can only attack a person within a certain range of your own notoriety level.
The problem is explaining why you can only attack certain noterieties, I suppose it could be a common law respected by all criminals.
Mabey there's an "underworld" mafia like society that makes the rules.
I'd also suggest that these quests would change. Perhaps the "contacts" have to keep moving in order to keep away from the law.
The nature of these quests should be pretty straight forward. Kill a target mob (or player with a certain notoriety), steal from a mob/pk player, poaching endangered animals, and delivering packages.

Okay, so we have a way for some players to "earn" the ability to attack eachother.

Now, here's a twist, pk players can sometimes be attacked by regular players.
Let's say certain players join the town/city guard.
Now, the quests a pk player does to earn notoriety involve certain conditions. For instanct, a poaching quest can't have any witnesses.
The same with an assasination. You can't get caught stealing.
IF you fail one of these conditions you get a warrant placed on you.
Guards and player guards can attack a player who has a warrant placed on them. These acts can have a pretty short statute of limitations, but when that's up you could have a bounty placed on you (if you have a rather high notoriety). Any player could collect a bounty.
I'm thinking that being taken in by a guard only results in a loss of gold (bail), and perhaps a curse that limits stats/movement/ and other random effects.

This seems harsh on pkers at first glance, and maybe it is. But I'm thinking that it might be simpler than I think and could prevent most forms of griefing, and weed out the players without the skill or will to truly play on the darker side of things.

Plus, I might like playing as a guard who has to do more that fight hordes of mobs. You might make them extremely clever and intelligent, but it still doesn't match up to protecting the town from a truly dangerous foe.
I'd like to be able to hunt down the deviants, as well as be hunted.
(I know, it sounds like I'm contradicting myself, I suppose I can only say that the thrill of the hunt can be much more fun than the reward of the kill.

Just to clarify, Players of a very HIGH Notoriety could attack and kill Guards. This act could come with interesting reprecusions, perhaps it could trigger an influx of monsters/invaders, as the defences are weakened. Like singular monsters coming in town.
But this could also be bad for the victor, as he could gain an imidiate bounty.

One more thing, perhap Notorious players could only attack others if they somehow identify them.
Seeing them commit a "Notorious" act, or even looking at some big criminal dociay.

Ah well, pick apart this long rambling post. Use it, abuse it, and change it how you want. Just see if you can pick out any good ideas.

You know, like in splinter cell or elder scrolls (just to give a general idea for a starting point, not a copy of either system) Here it could be more interesting.
Your skill level in picking could be represented in a huge number of ways.
At it's simplest level you use certain picks to manipulate tumblers.
But as the difficulty of locks increase you start to enter a more strategic game.
At the highest level locks I imagine that you would essentially be manipulating a big Rube Goldberg Device, disabling certain traps and clamps that trigger others. Some could set off nearby traps, or small traps that send out poisonous fumes or darts, or clamp onto your pick, or that simply jamm, disabling the lock entirely, until a locksmith can change it out.

This same principle can be applied to traps as well, only it could be used on a slightly larger scale.
This way there would some deal of individual skill and drive necessary to create as well as "destroy" or collect these devices.

I imagine that this would make the making of these devices just as fun, or moreso that the disabling of them.

As far as traps go, I imagine that there would be size issues involved that would allow there to be small self contained traps for hunters who don't want to bother with guarding a guildhall or treasure trove.