Steal right now can do two things, either steal from a specific "steal" pool with low rate of success or dispel a buff from the mob and potentially apply it to yourself (with a high rate of success if merited). I suggest adding another layer. Steal should be able to steal from the mob's normal drop items and drop the item into the treasure pool at some appropriate rate of success (not too high as to be unbalanced, not too low as to be useless).

Alternatively, give this additional ability to Mug instead.