Infiltrators might be able to kill captains.
Though the idea about searching for secret societies is something that quite literally sent chills down my spine. Just to be able to locate some, to learn the name of the society and where it is, that alone would be very very nice. Though naturally one could expand on that, like an extremely rare ability to get a letter sent to the society or some such.
Anything that makes the class more towards spymaster / intrigue type of play is good in my book.
Perhaps there might be a kind of an "assassination light" button that does not wound you but just makes you lose some or all hours at the turn change. So whatever you thought you had enough time to do you suddenly do not. Someone messed your papers, or the negotiation you entered took longer than expected, or the wine you drank just made you incredibly drowsy. You could get even more variables by choosing to either target one character or everyone in the region. A bit like messing with them signs, but more broad usage.
Also, you could "intercept scouts", which would basically mean that anyone sending scouts to your region stands a significantly higher chance of never hearing of them again. Though this would be pretty much a defensive act and perhaps not even illegal. Though it could still be risky, if the scout fought you back and dragged you back to his master's dungeon.
Or you could confuse scouts by presenting them with reports of a significant portion of the troops missing.
Your own scouts, of course, would be less prone to such maneuvers, depending on your skill.
Also, you might harass food trades. For the easy jobs, the food would just go missing for some time. For the hard jobs you could make it look like the lord sold it to the enemy or to the black market (if the feature works). As a bonus it would add some plausible deniability to lords actually selling to someone they should not.
Perhaps have some actions where you may manipulate the stats of a region by creating scandals.
Maybe just an option to create scandals to hit the target's honor and prestige?
Sorry for the wall of text. I just love the class and would like to see more use for it, especially with intrigue.