I think you could work another class into the same category as monks/rogues...
Make it a "Nightblade" (or something else Oblivion-esque), no pickpocket, reduced damage on backstabs but adds a lot of utility such as different poisons such as snare, shaman/enc low level slow, malo (sort of borrowing from WoW there), or even some DDs - like gnomish fireworks but that will cause damage that would put them on par with rogues, or increase their damage potential and give them leather armor but not mail.
...that's what I'd add, think it would be a lot of fun
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