I have ~1500 hours and have never done any of the bookmarks except 769. Most of the stuff people that people are saying are "rare unicorns" are no more rare than the things that they list as "common". I don't think there is much you can count on being consistent in the 769 bookmark between playthroughs. Byz is consistent to some extent but even they can be taken out by hordes or turn into a theocracy, etc. I've seen AI Francias, HRE's, Miaphysite Egypt, and reformed pagans more times then I can count on my hands. And that is just the stuff people have mentioned in this thread.