I semi-accidentally discovered a fun thing with Shattered Worlds. I'd left a Shattered World running in observer mode and forgot all about it... until late in the next day, when I noticed CK2 was still running and that the in-game year was 1912. The amount of rulers and history was incredible. So much had changed in the world, but some early realms were still there and recognizable. The border gore was probably the worst I've ever seen, though. I'm trying this again with exclave independence rules much harsher, which will hopefully lead to a more sane world. I'm also sticking with regular religions and cultures, just shifted around, plus default titles for a more recognizable-yet-different kind of world. I'm going to convert it to EUIV when I'm done. This is a cool way of making new, different starts for EUIV and also just seeing how the world shapes up so many centuries, much like DF's Legends mode.
Things are already shaping up nicely. My money is on the Alan Slavic kingdoms in Italy. I hope to see them dominate the continent by the end. The Cathars of the Middle East and the Finns of England are also favorites. Oddly enough, India ended up populated mostly by Indian cultures anyway (they're all Zunist, though).