Birken said:

A new faction that can unite vassals against tyrannical rulers, change succession law and put the most liked (or the least disliked) vassal on the throne. Greetings! Click to expand...

Like elective monarchy in every freaking kingdom due to factions revolting wasn't enough... First of all, I highly disagree with "the most liked" vassal being put on the throne. If he doesn't have a claim to the throne, he should not be able to take it. How often were kings toppled by someone with zero claim to the throne during the middle ages? Pretty much only happened in Byzantium (and the HRE, but since the HRE was elective, it's a different matter), which was a unique case due to their history of being a republic and had a long history of coups. Second, you should fix the AI's behavior first, because a ton of my vassals are having issues with rebellions. At least let the player intervene in those cases and either 1) end the hostilities through diplomacy (I'm not sure if realm peace affects that, but I don't think so, right?), 2) join your vassal in war against his rebels. Because as it stands, the AI stinks at handing out duchies and holdings and incur a lot of opinion penalties strictly through simple things like having four duchies while being a king when they could be handing out vice-regal titles.