My PoD of the Day - no Phocas, the Rashidun conquers Persia and moves into Central Asia too. It eventually splinters between Arabia, Persia, and Central Asia. Meanwhile the Romans create an Exarchate of Taurica that focuses on Steppe/Cossack style warfare, which backed by Roman money and resources comes to dominate the Steppe, before Roman forces unite the Rus under and Exarchate or two.
A different one is less PoD of the day. The Emperor, after seeing the Varangians and creating the V.Guard, sends a number of priests north into the Rus, specifically to ally with and convert the Kievan Rus and other polities. The Romans support the Kievan Rus in a joint treaty to prevent any steppe tribes from crossing the Dneiper, leading to a large number of Roman and Kievan forts, alongside Roman and Kievan settlers, who intermingle to form a hybrid culture. Inevitably conflicts arise over the territory, but those towns reliant on trade between the Kievan Rus and the Romans sell cold-weather gear to the Roman forces to make it easier to fight, as they march north to Kiev, winning a victory and making it into a number of smaller client Kingdoms, and taking the hybrid territory as a province. Over time, the slightly wealthier (i.e. Greece isn't repeatedly ravaged) Empire uses some of those resources to defend Anatolia more successfully, preventing Turkish conquest. The Rus Clients are lost when the Mongols invade (although the hybrid territories are successfully defended), but as the Khanate falls apart the Romans re-establish the Kievan Client state, and it becomes the rival of Moscovy - whilst Kiev tried to bring more territory under Roman rule (and thus thrive on their share of the tributes), Moscovy does the same for the Mongols. By this point the Kievan Russians are practically Roman (and are accepted as such), leading to a Kievan dynasty that not only dominates the Russias, but rules it from Constantinople.