Something that annoyed me in my Sengoku games so far is how relatively powerless you are against treacherous vassals despite being the clan leader. If a powerful vassal is really commited to ruining your day, there's not much you can do about it. You can try to lower his honor using ninjas, but most of the time you do it once (which is usually never enough to get him to commit seppukku) and by the time you can get a second ninja he already has more honor than he had at the beginning. You could try to kill him, but chances are his successor will hate you even more. And reducing his power by taking back his titles is horrendously expensive in honor (I know, I know, honor is plentiful at this point, but the nice fellas from Paradox are supposed to be working on fixing that for the next patch). So I have a couple suggestions to give you more options in dealing with these damn traitors.
First of all, how come they can plot against you, send ninjas against you, and do all kinds of mischief and then come out unscathed even if they get caught? Why would it even cost me any honor to take titles away from some jerk who just allied with my worst enemy to betray the whole clan? So I propose to bring back the concept of "casus belli", just not against other clans (opinion works well in that case), but against your vassals. If a vassal just tried to kill you, you should most certainly be more than justified in taking all his titles and making him split his belly, so the honor cost in taking these actions should be reduced. Discovered plots, ninja attacks and things like that should give you a "justified cause" to take actions against your vassal. And the concept could be expanded even further: Imagine a new ninja action, let's call it "Framed!", that on success would give a clan leader a "justified cause" against a certain vassal of your choice...
Other things that you could do: trying to remove the vassal by force in case of extreme disobedience or rebellious behaviour. Why is it that only a vassal can trigger a civil war? The clan leader could also spark one by sending his army to remove a vassal forcefully. This would immediately give the rest of your vassals a relations penalty against you (if you don't have the aforementioned "justified cause") and then create a civil war where you get the vassals that support your heir on your side, and the vassals which support the victim if he is a pretender or would have supported him if he was (after taking into account the relations hit) on his side. This way, if you saw a rebellion coming, you could spark it on your terms (instead of resigning yourself to get backstabbed the moment your powerful neighbour finally decides to gobble all your lands).
Finally, how about plotting against one of your vassals? Not sure how this could go, but maybe if you can get most of the clan against him he could be forced to renounce to his claims to clan leadership and/or retire to a monastery. I just think plotting is a great mechanic, but a bit underused at the moment, and it would be great if a clan leader could find more uses for it than finding allies for attacking a bigger clan.
First of all, how come they can plot against you, send ninjas against you, and do all kinds of mischief and then come out unscathed even if they get caught? Why would it even cost me any honor to take titles away from some jerk who just allied with my worst enemy to betray the whole clan? So I propose to bring back the concept of "casus belli", just not against other clans (opinion works well in that case), but against your vassals. If a vassal just tried to kill you, you should most certainly be more than justified in taking all his titles and making him split his belly, so the honor cost in taking these actions should be reduced. Discovered plots, ninja attacks and things like that should give you a "justified cause" to take actions against your vassal. And the concept could be expanded even further: Imagine a new ninja action, let's call it "Framed!", that on success would give a clan leader a "justified cause" against a certain vassal of your choice...
Other things that you could do: trying to remove the vassal by force in case of extreme disobedience or rebellious behaviour. Why is it that only a vassal can trigger a civil war? The clan leader could also spark one by sending his army to remove a vassal forcefully. This would immediately give the rest of your vassals a relations penalty against you (if you don't have the aforementioned "justified cause") and then create a civil war where you get the vassals that support your heir on your side, and the vassals which support the victim if he is a pretender or would have supported him if he was (after taking into account the relations hit) on his side. This way, if you saw a rebellion coming, you could spark it on your terms (instead of resigning yourself to get backstabbed the moment your powerful neighbour finally decides to gobble all your lands).
Finally, how about plotting against one of your vassals? Not sure how this could go, but maybe if you can get most of the clan against him he could be forced to renounce to his claims to clan leadership and/or retire to a monastery. I just think plotting is a great mechanic, but a bit underused at the moment, and it would be great if a clan leader could find more uses for it than finding allies for attacking a bigger clan.