I'm just finishing a Byzantine game, up to 1447. Started as Doux Konstantinos Argyros of Samos in 1066 and decided to be the power behind the throne, rather than Emperor. Supported the Doukas-lead Empire for a hundred years by leading holy wars against any Muslim or pagan leader (Cuman, Fatimid, Seljuk) that launched jihads or holy wars against the Empire and by leading claim wars against Greeks revolting against the Emperor, forcing them to fight two front wars often leading to white peace with the Empire and a little land for me. Eventually crowned myself King of Taurica.
In 1190, as the Despot of Taurica, founded a faction to press the Queen of Georgia's, Romilya Doukas, claim to become Empress. She happened to be married to an Argyros, with Argyros heirs. Thus started the Argyros line of Emperors that lasts to this day. The Taurican Argyros supported the Emperors against all foes foreign and domestic, except for intrafamily squabbles for the throne. As long as an Argyros ended up on the throne, I didn't care which one.
This has lead to Byzantium being the most powerful Empire in the world, and Taurica its most powerful vassal. Been very fun game, now need to decide on where to start next.