So you would want Iraq,Western Iran,Kurdistan regions,Armenia,Georgia,Levant,Egypt,Caucasus,Central and Eastern Anatolia in the ERE and everything else is controlled by the WRE? So the capital probably would be on the Levant or the Mesopotamia to be safe. Armenians,Caucasians,Scythians/Alans/Sarmatians(Ossetians),Western Iranians and Georgians would be considered Roman along with the OTL Levantines,Mesopotamians and Egyptians.
This empire would be diverse,stable and richer as I see it. It would pour incredible wealth for the Empire and provide protection against,Germanic and Slavic invaders so they are now in the WRE and not ERE. But ERE will have to face the Turks and the Mongols later. Islam could be butterflied in the Northern Middle East(Iraq,Syria,Lebanon,Palestine,Turkey,Iran,Kurdistan and Caucasus. Instead,it could expand into Africa but wouldn't be as large as OTL. Both WRE and ERE will endure for a longer time in this timeline. Middle East and Eastern Europe would be very different from OTL. Hungary could end up existing where it is whereas the Slavic countries borders could be butterflied.