I was kicking around some unrelated ideas and a few random thoughts wandered through my head. Given the present state of things, they might be cool. Anyway, some thief skill ideas...
Basically, these are the opposite of the resist spells.
Alcohol bath: Would give alcohol an actual use in the game besides talking funny. May or may not use the entire container... Requires some alcohol to work, the more drunk it makes you, the more it weakens enemy saves to fire magic. So, beer might not help at all, but a little whiskey could weaken saves against fire magic. Let mages light em up more often without 1/2 damage.
Water bath: Requires a canteen to work, uses the entire contents of the container to work. The more water used, the more vulnerable they are to lightning and cold damage. A canteen will only raise their saves a little bit, a water barrel raise them a bit more. Interesting side effect could be to grease the room if there's enough water and a frostbolt.
I didn't think of anything for acids but acid has an effect of it's own. Colors and disints? No ideas. Oh well.
I can't really picture this working well on stab/blast mobs... unless there's a second non-stabbing, non-blasting thief in the group. Also, if you raise fire magic on a mob that casts fire magic... it might backfire on the group. Would give mages a boost on ether mobs like the eternal flame or flamewraith, or on mobs like Paraclesus that hit like a train no matter how much you debuff them. Would also give thieves something useful to do in a group after stabbing besides drain their moves with circle.... at least for a round.