I have several ideas in regards to these type of classes, since they are the ones that call my attention most. Call it a Ninja, a Rouge or an Assassin, they pretty much act in the same fashion. Sneaky little SOBs that sneak about stalking their targets and ambushing them. In here, I will mention some of the skills and traits the class(s) may have for the future.
Name: Stalk
Effect: Allows you to stalk a target. It would rather much work like your avarage 'follow' command, only the person will not recieve the message of '$n now follows you.' or whatever. With Sneak and hide, this skill could be very useful. Also, Stalk would be a requirement to be able to activate other skills, such as the next one I have in mind. Another thing, while stalking a target, I would anticipate it would consume a bit more stamina upon movement, but no delay (lag), because of course, you are following (stalking) them.
Name: Ambush
Effect: With stalk activated, the user will be able to ambush their target, causing an automatic 'critical' blow to the target. A simple critical hit upon activating is more than enough to be the effect of this skill. Automatically upon using Ambush, the effects of stalk would wear off, and combat would have of course been initiated by then. I believe the skill would be based off agility, and would consume a decent amount of stamina according to the agility level of the user. This may also determine overall damage for Ambush.
Name: Pick-pocket
Effect: C'mon... Not much to share about it. You're stealing from someone's inventory/money pouch!
You should be able to pick what you are willing to steal from, the money pouch or the targets inventory. The max allowed percent of success in this ability should be a 50%. But along side Intelligence and Agility, it should improve (lean) more torwards success. Inventory items having a higher chance of being more valueble, it is more likely to get caught stealing, however, the money pouch should be less more difficult to rob. The syntax could be 'pick-pocket $n inventory/money' or something similar. Also, to allow a chance at retaliation, the skill would have a delay. Giving the target a chance to attack the user. This is one of those skills in which the user MUST be lethal. But I don't think it should also mean the target has to be lethal also... But then again, that's just me. I always thought people could fight and always have a fair chance at anyone in this game. Whichever the case may be, give it some thought.
Sneak and Hide could be moved to this chain of classes also. Since it is more fitting, along with Poison Weapon and Conceal Weapon.
As for class level ups, I think Stamina would be the main bonus for this class. That, and little to no Health bonus. Whichever.
And as for a cool trait, while A ninja / Rouge / Assassin, you should drop less Yen than usual when you flee, that, or none at all
Anyways, that's all I got for now.