So, got some more ideas for this.
First, let me ask this: how bad would it be if some ability had a percentage to trigger, and it offered a save? Something like "whenever <class> attacks, there's a x% chance <something will happen>"?
For the classes I have in mind, I would need some way make a RNG to trigger the effect (and it would grant a save). Maybe if it rolled a d6 and picked some range...