Warluster, the Byzzies were my favorite faction in BI, I remember many bridge battles against the various hordes along the Danube.

I also read John Julius Norwich's trilogy on the Byzantines, so I would have some historical background on them. It'd be fun to see if I could avert the decay and downfall of the ERE after Manzikert. Have my own Komnenian restoration.

Of course those books did give me some background on the Seljuks and the Muslim invasion of the Empire. So Egypt and the Turks would be interesting in their own ways.