Well, I finally went back to my Charlemagne game. Charles ended up dying one year later than OTL: I'm pretty sure I didn't conquer as much territory as he did (Bavaria is still independant thoug allied, there is no Marcia Hispanica and I never went to war with the Avars). That said, I think the biggest challenge is yet to come for me.
For one, Charlemagne only had one son in my scenario: Pepin the Hunchback. The Hunchback however pretty much died like OTL though he left one son behind who succeeded Charlemagne: Robert I, Holy Roman Emperor, King of West, East and Middle Francia, King of Aquitaine, Burgundy, the Netherlands, Italy and Saxony. A thirteen year old whose current regent is the Doge of Genoa because I was unable to find how to appoint a designated Regent.
Family-wise, the next-in-line for every throne save maybe Italy is Carloman's greatson: Pepin. Whom I had tutored as Charlemagne and who is not only one of the most competent but also one of the nicest persons around: he has five of the seven virtues and is best pals with his cousin Robert (which is why I'm planning to make him Robert's tutor). Then I believe comes a more distant relative who is the current Duke of Flanders.
I'm a bit worried about the succession right now: Regency periods are always a nightmare in CKII though fortunately this one will be rather short (a bit less than three years). But Saxony hasn't fully converted to Catholicism (though most of its lords did) and most of its lords are Tribals and thus hate Feudalism. The Pope is also not to happy with Robert at the moment, though that probably has to do with the fact that I own two counties he wants control over. To remedy to this situation, I'm considering handing over the Kingdoms of Saxony and Italy to vassals.
Speaking of which, the vassal limit is a bit challenging to manage. I'd like to centralize but I currently can't because I'm really at the limit of the number of vassals I can have. Maybe that's because I'm not ready to grant Kingdoms to my vassals at the moment though. And Feudal revolts sometimes lead me to go over the limit until the revolt is over. Still I like the mechanic though it probably needs a bit of a rework.
A few questions:
1) Can anyone tell me how to designate a Regent for my Empire?
2) How can I vassalize the Pope without going for the Antipope route?