When I have a pretty large empire and the whole thing is getting predictable, to the point where I'm ready to start again, I often take a look at my inbred loon of a king and his line of pathetic heirs and then take a look at some of my top generals and think: "it's time for a change". This is known as orchestrating a civil war:

1) Build several (2 - 3 should be enough in fact) crusade/jihad markers in one province.

2) Find all of the very loyal and worthless units and dump them in the expendable provinces or place them in separate stacks facing larger stacks, where their revolt would be disastrous.

3) Find the best high loyalty generals and start trying them for treason with 0 valour spies. This is not guaranteed to work and in the worst case one of you spies might pull off his mission.

4) Demolish chapter house and hold on tight (don't forget to back the rebels - no matter what!).