CK2, for all its problems, had a pretty sensible way of representing the Byzantines:
· the empire wasn't hereditary, and often changed hands between dynasties
· the Byzantines could actually lose land (I think I saw Muslim Anatolia in about 20% of my games, and I once saw the AI Seljuks create the Sultanate of Rum when I started in 1066) instead of being an unstoppable purple blob
· the Byzantines could, and did, expand in sensible directions instead of deciding to conquer Khazaria or Hungary for shits and giggles
· duchies weren't hereditary and could be revoked, making them much closer to Byzantine themes than feudal lordships
Sure, there are a few things I'd change (Orthodoxy existing before 1054, Greek culture being a monolithic blob, Byzantine AI rarely choosing to reconquer anything in k_sicily or e_arabia even if it could easily do so, the HRE not being able to restore Rome) but overall, the Byzantines are a lot more historical in CK2 than CK3.