So I was Emperor of Hispania again looking to combine it with the Byzantines again. I matrilineally married my daughter to the first-born of a guy who had a strong claim on Byzantium, after which I promptly declared war on Byzantium and installed him. The plan was to marry the Byzantine branch with the main branch a few generations later so they wouldn't be genetically similar. What I noticed, though, was that the Byzantines decided to elect the new emperor's third-born son who was NOT matrilinealy married to one of my daughters. I noticed this after my emperor died and my new emperor had to deal with two civil wars and a peasant uprising despite having good stats and traits.
I was very fortunate, however, because early in the reign of my previous emperor, I installed my antipope as the real pope and said new pope was of my dynasty (appointed by me thanks to Free Investiture.) This made the pope a vassal of me. I mostly used the pope in this game and other games to grant divorces and excommunications on loyal vassals, but then I realised that I owned the pope not merely the anti-pope. This meant I could excommunicate, claim titles, and - most importantly - request invasions on every Catholic in the world. Luckily for me, my previous emperor forced the new Byzantine emperor to convert to Catholicism while in court so he could catholicise the Greeks while we waited.
Cue request invasion against the Byzantines and a successful invasion after it was automatically accepted. Since I can claim any non-independent titles too (or at least duchies), I shouldn't have any problem with taking Ferrara and all the other Italian-held duchies crucial to restoring the Roman Empire. So long as I keep my Piety high, I can excommunicate any problem vassals who may wrongly choose to vote for people who shouldn't be emperor.
Seriously guys. If you're an emperor, get a dynasty member as antipope and install him as the real pope. It's so worth it.