Here are some new game mechanics that might fit a thief class.
Case
Besides giving a list of items on target, it also can return a list of memed spells.
Taunt
Castors spells are diverted as if confusion is cast. Works with specs.
Also, besides switching target to thief, mobs affected will follow thief out of the room leaving their post.
Treasure find
Increase chance of spell load, coin drop increase and chance of rank point increase
Entrapment
1d4 rounds to setup, mass entanglement with the following effect. One setup per room.
1. Whiff
2. Trip and sits
3. Trip and hit own group
4. Trip and hit self
5. AC=0
Damage demon
Damage + 1 stacked for consecutive non miss and same target.
When target dies or change, damage is reset.
For example, a thief will have +10 dam after 10 non miss hits while in combat
Envenom
I think envenom should be more controlled with different types of poisons
1. Temp blind
2. Temp silence
3. Temp paralysis
4. Memory loss
5. Slow
6. Target cant be cured for 1d3 rounds
7. Weaken with chance of fumble
8. Confusion