This is so much more convoluted than just giving the Byzantines their own civ. I can't even imagine how it would be implemented and not confuse players. "Here's this alternate leader for Rome! Except the unit, district, city names, and icon are all different because of...reasons." The fact is the classical Rome focus of the Rome civ immediately jumps out as anachronistic and out of place to players when mixed with a Byzantine leader. If a separate civ bothers anyone that much it's simple to edit the text to say Rome instead of Byzantium.