if they learned the art of poisons and deceptions?
Like stacking poison effects from processed monster parts. Such as wamoura blood coated onto a dagger inflicts a burning effect. The more you attack the more potent the poison will become as it creates infected areas. Maybe even coatings that can slow enemies down as their joints stiffen. Or a poison that causes an enemy to become drowsy and can force them asleep after a small time has passed. Maybe use sneak attack to apply multiple stacks and encourage using all tools?
Covering the line of debuffs not covered by dancers while not looking to step on the enfeeblement master, rdm. You can have two types of poisons, the debilitating which cause status debuffs and the facilitating which add directly to the thief's damage.
I'm not saying however that the debuffs should be stronger or equal to a rdm's enfeebles yet they should find a solid middle ground to be between, say, slow I and slow II. They could even find a niche as a physical based debuff instead of magical based possibly as a means to override an monsters immunity vs a slow or paralyze. The debuffs could still come in tiers such as the difference between a damselfly poison and a wamoura poison.
What if hide was changed to be a buff until your struck(any action made towards you); Adding an attack bonus and/or critical bonus as long as your not the focus of the attacks(becoming more potent on your current position and possibly time hidden)?
Maybe adding an ability like a stun that works off of accuracy. Yet unlike a dnc stun provides a boost if you interrupt during a spell or an ability. Increasing all forms of damage taken by the target. Make it a low cool down(1min-ish). Call it something like thwart or disrupt.
It's been in my head for a while. I always felt the thief/rogue/assassin types of classes usually add some level of these styles to cripple enemies while staying the shadows while making the next strike twice as devastating than the previous.