You could look at:
- the Goths and their influence on both the politics and theology (esp. Arianism) of the Empire
- the role of the bureaucracy
- how the political struggles between the Empire and the West affected ecclesiastical struggles/conflicts (esp. important in Illyricum)
- Justinian's attempts to recapture the West
- The Codex Iustianus and its influence (or lack thereof) on modern/classical Western jurisprudence
- Basil II as the height of the Empire
Do yourself a favor and get Ostrogorsky's
History of teh Byzantine Empire. It is fabulous. Also very good is J.B. Bury's
History of the Late Roman Empire