Plot system

Successfully plotted only once, against beaten up Uesugi. Just clicked "Invite to plot" button and some two guys in the list were game. Declared the war, activated the plot. Turned out the two guys were Uesugi vassals owning about 80% of clan territory. They changed sides, the remaining provinces were torn apart by neighbours.

Honor is just a factor in the total of opinion modifiers. Succession crisis in the clan, denied titles, gifting, relations improvement can be of help. Many refuse to plot if you're overpowered already

In my experience, the power of your enemy is one of the keys for successful plots. No one seems to join a plot against OPM, or 2PM. On the other hand, strong clans such as Uesugi, Yamana or Hosokawa will be good targets.

Yes, big clans that touch a lot of other clans can be good targets. I've used plots a couple of times for those clans that are a bit bigger than me. Keep checking back to plot invite occasionally because it can change with lots of new folks receptive to an invite all of a sudden.

well you have to work on your plot. dishonoring and assassinating right targets.
http://forum.paradoxplaza.com/forum/showthread.php?564719-Show-your-mastermind-plots&highlight=