Well, some bookmarked characters have the very hard tag, that's a good place to look at.
The infernalist nosferatu, Lukas, is often credited as the most difficult bookmarked character. Though it's obviously debatable.

Well, difficulty tag reflects on amount of external threats character would have to face. It would not be much of challenge to play in hostile enviroment for hardcore inquisitor.

I'd suggest to look for characters you personally enjoy playing, rather then looking for mechanical challenge. I don't think you will find any at this point. If you have no favorites you want to play from the lore, just click around and read starting descriptions in hopes it would spark interesting ideas for playthrough.

I personally love "very easy" Camilla and "very hard" Etrius equally, while Mithras and Lukas equally bore me to hell. Lukas is just annoying btw. If I was forced to play standing, rather than sitting, it would not bring my skills to unbelievable heights. It just would make me uncomfortable.

It's not complaint though. His role is to fail terribly and serve as an important unlife lesson. With that in mind he is perfectly balanced just like any other bookmarked character.

Hey ! Just be like me, start every game by having every vassals turn against you in less than a year, lose every piece of territory you have and be independant with just 1 county VS the world (Ya, no win :D)

You can do that by creating a custom character, any race you want, pick the tribal ones (Sisters of the Fury, or whatever the name of this thing is), and try to survive in england, or france, or anywhere really without having your stupid vassal turn against you instantly x)

It would be the same as creating a custom Muslim character to lead the Holy Roman Empire in vanilla. That would probably go very poorly, very quickly.
It's certainly a way to increase the difficulty at the very least.

@SHADOW
First, by playing custom character you forfeit your bragging rights.

Second, most of vampire vassals you get to start with are indeed liability rather than a boon. The challenge is to keep them in line while they are still useful and replace them once they are not. There are starts where controlled revolt is perfectly good strategy to get rid of the worst troublemakers in conflicted realms. But controlled is the key word. And it makes game easier, rather than harder. It looks like you simply mismanaged your vassals, whish is not an accomplishment. Git good.