There are a few great topics about a Byzantine Empire / Roman Empire civ concept here on the AoE4 forum. You can use the Search buttton and so a little research, maybe build your own concept, would be much appreciated.
Also check out my AoE4 DLC themes investigation and poll:
Also see the “Rise and Fall” analysis of individual civs during the time of AoE4, including important historical snapshots of the Byzantine Empire: