All countries playable crusader kings 2 question

[Emperor Justin]

Is there a mod that allows you to play as all countries regardless of their government? Like republics or the papal states?

[Veliky Kaiser Theos]

Not that I know of, and there wouldn't really be much point. When you die, a random character will inherit rather than a member of your dynasty, so the game would be short. I've tried playing as the Papal State by winning a war to depose my liege as a vassal, and I couldn't change inheritance laws or create a kingdom to set as my primary title, so your game will probably be short if you do find a way.