I know alternate Constantinoples have been discussed before, but I think these two have the same benefits. As islands, they are protected from invasion, and aren't quite as developed as other cities, making it a blank slate, plus they are both in the middle of the empire, and the center of respecetively the ERE and WRE, more or less. If so, which one? Would it move? Could they both be the capitals of the West and East after the split? Would there be a Patriarchy of Knossos?