Nicomedia in Asia Minor was the capital city of Diocletian when he created the Tetrarchy; his Caesar, Galerius, was based at Sirmium close to the Danube frontier. It is also a possibility that Licinius briefly made Byzantium his capital in case Constantine invaded the eastern empire just before the last round of the civil war between the two emperors (according to P. Stephenson).

Edit: Also worth adding that the imperial capitals in the west at this time were Trier and Milan - the city of Rome remained important, but it was no longer where the imperial court was based.