I've been getting a real kick out of the "Dynasty of Many Crowns" gameplay. I don't actually care about achievements - which is why I usually start with a custom character and cheese certain things for my own personal roleplay - but it is satisfying gradually setting up dynasty members on foreign thrones. My current game has my MC as King of England, Wales (technically - I never created the kingdom title, just let Gwynned conquer the whole lot then renamed it "Duchy of Wales" since "Principality" isn't a thing), and Ireland, with relatives on the thrones of Scotland, Denmark, Galicia, Brittany, Pomerania (via redirected crusade), and Sicily.
My only regret was getting involved with the Iberian Struggle. Constantly being called to war, and there are bigger penalties for declining now than there used to be, even though the King of Galicia has more than enough gold and troops to fight most battles on his own.
I've also been switching characters to create hybrid Anglo-Saxon/whatever cultures in certain places. Like Pomerania is now "Pommish" and the people are "Pomms". They speak Letichic with a West Germanic heritage. I keep getting dragged into wars there, too, actually, since my character's brother, the king, has been aggressively converting religions (he's got a good bishop - 29 learning isn't bad at all).