Something similar for thieves and assassins. A disguise skill that removes them from where while sneaking, and makes them look like a comparable mob in the area. Like if used in Galadon, they could disguise themselves as a cityguard or merchant or whatnot. Give it a chance every tick that the disguise slips, or someone sees through it and the whole thing is ruined. Maybe make it so you have a lower chance of a successful disguise the further away from the target mobs natural appearance you are. IE felar would have a harder time disguising themselves as guards in Galadon, but easier in Blackclaw, humans would be the reverse, etc etc. It would fit in more with the classes that would be known to be capable of disguising themselves.

Maybe make it so stealing and assassinating have a chance of being hindered by the disguise as a way to balance it out, again based on the difference between your natural appearance and whatever you are disguised as. With proper drawbacks like those above and maybe a timer comparable to request lag while you "don your disguise", I think it could add a lot to those two classes. Maybe make it so you have to have a certain amount of the target mobs eq on, or something. You have to actually be wearing cityguard armor to look like one, etc. Just something to toss out there. Nothing as powerful as old school dopple, but something that might add more flavor to the "sneaky" classes.