You've got a problem then--Bulgaria.Those guys didn't even bother to make excuse that they want to become the ERE's successor state.The Bulgars were pretty much at their heyday during this period.If the ERE fragments completely then the tiny ERR is most likely gonna be overrun by the Bulgars to the north.
But not the city itself. On the one hand, the city is pretty impregnable at this time, withstanding repeated sieges from far superior foes. Plus, the Bulgars actually sided with the Byzantines against the Arabs during the 717 siege.
That said, I see the Bulgars definitely expanding their territory during this.